Route hand-rolled diagnostic pragmas through Hedley

Several places in the library hand-roll compiler diagnostic suppression
with raw `#pragma`/`#ifdef __GNUC__`/`#ifdef __clang__` guards instead of
using the Hedley primitives already bundled and used elsewhere
(JSON_HEDLEY_DIAGNOSTIC_PUSH/POP, JSON_HEDLEY_PRAGMA, ...). Converted six
of the seven listed push/pop pairs to use those primitives instead of
raw `#pragma GCC diagnostic`/`#pragma clang diagnostic` text:

- include/nlohmann/json.hpp (~3770, ~3863): -Wfloat-equal
- include/nlohmann/detail/conversions/to_chars.hpp (~1078): -Wfloat-equal
- include/nlohmann/detail/output/binary_writer.hpp (~1844): -Wfloat-equal
- include/nlohmann/detail/iterators/iteration_proxy.hpp (~211): -Wmismatched-tags
- include/nlohmann/detail/exceptions.hpp (~36): -Wweak-vtables

iteration_proxy.hpp did not previously include macro_scope.hpp itself
(it only compiled because some other header included earlier in
json.hpp happened to pull macro_scope.hpp in first); it now includes it
directly like the other detail headers that use Hedley macros, so it is
self-contained.

Each push/pop pair now uses JSON_HEDLEY_DIAGNOSTIC_PUSH/POP
unconditionally (a no-op on compilers that don't need it) and wraps the
actual `#pragma ... diagnostic ignored` text in JSON_HEDLEY_PRAGMA so it
goes through Hedley's _Pragma()-based emission instead of a raw #pragma
line, while keeping the original `#ifdef __GNUC__` / `#if
defined(__clang__)` guard around the ignored-pragma itself.

Deviation from the issue's suggested transformation: the issue's example
replaces the `#ifdef __GNUC__` guard with `#if
JSON_HEDLEY_HAS_WARNING("-Wfloat-equal")`. JSON_HEDLEY_HAS_WARNING is
implemented purely via Clang's `__has_warning` builtin and evaluates to
0 on real GCC (`#define JSON_HEDLEY_HAS_WARNING(warning) (0)` when
`__has_warning` is not defined), so adopting it verbatim would silently
stop suppressing -Wfloat-equal on GCC -- a real regression, not just a
style change. The existing `#ifdef __GNUC__` / `#if defined(__clang__)`
guards were kept for the ignored-pragma to stay behavior-preserving, and
only the push/pop/pragma-emission mechanism was routed through Hedley.

Two of the seven locations from the issue (the -Wignored-attributes
push at the very top of json.hpp and its matching pop after
`#include <nlohmann/detail/macro_unscope.hpp>`) were intentionally left
unconverted:
- The push, at the very top of json.hpp, runs before
  `detail/macro_scope.hpp` (and therefore hedley.hpp) has been included
  anywhere in the translation unit, so JSON_HEDLEY_DIAGNOSTIC_PUSH is not
  yet defined at that point.
- The pop runs after `macro_unscope.hpp`, which -- via hedley_undef.hpp
  -- has already #undef'd every JSON_HEDLEY_* macro (by design, see
  #5408) precisely so they don't leak to users, so JSON_HEDLEY_DIAGNOSTIC_POP
  is no longer defined by the time the pop is reached either.
  Making this one pair work would require either hoisting the ~2000
  line vendored hedley.hpp to the very top of the amalgamated single
  header (a much bigger structural change to single_include than a pure
  mechanism swap) or special-casing this one pop ahead of the general
  macro cleanup. Both are riskier than the mechanical, behavior-preserving
  change requested, so this pair was left as-is.

## Validation

- Compiled include/nlohmann/json.hpp and single_include/nlohmann/json.hpp
  with `-Wall -Wextra -Wfloat-equal -Wmismatched-tags -Wweak-vtables`
  (clang, which self-identifies as __GNUC__ too): no warnings, same as
  before the change.
- Compiled and ran tests/src/unit-to_chars.cpp, unit-conversions.cpp,
  unit-iterators1.cpp, unit-iterators2.cpp, and unit-class_parser.cpp
  against the fixed include/: all pass.
- Compiled unit-msgpack.cpp, unit-bjdata.cpp, and unit-ubjson.cpp (which
  exercise binary_writer.hpp's write_compact_float extensively): all
  compile cleanly; the vast majority of assertions pass (the only
  failures are pre-existing environment issues unrelated to this change
  -- missing generated test-data files, not code correctness).
- Ran `make amalgamate`; the single_include diff is limited to exactly
  the lines touched in include/, with no unrelated reordering.
- No real (non-Apple) GCC was available in this environment to test
  directly; the `_Pragma("GCC diagnostic ...")` text emitted by
  JSON_HEDLEY_PRAGMA is byte-identical to the prior `#pragma GCC
  diagnostic ...` text, and the `#ifdef __GNUC__` guard is unchanged, so
  GCC's behavior is expected to be identical. CI covers the GCC matrix.

This PR is stacked on top of #5475 (issue-5408-hedley-undef-leak) since
both touch the same files; only the last commit here is new.

Fixes #5409.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -4944,9 +4944,9 @@ NLOHMANN_JSON_NAMESPACE_END
 // functions to. As a result, we suppress this warning here to avoid client
 // code stumbling over this. See https://github.com/nlohmann/json/issues/4087
 // for a discussion.
+JSON_HEDLEY_DIAGNOSTIC_PUSH
 #if defined(__clang__)
-    #pragma clang diagnostic push
-    #pragma clang diagnostic ignored "-Wweak-vtables"
+    JSON_HEDLEY_PRAGMA(clang diagnostic ignored "-Wweak-vtables")
 #endif
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
@@ -5198,9 +5198,7 @@ class other_error : public exception
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
 
-#if defined(__clang__)
-    #pragma clang diagnostic pop
-#endif
+JSON_HEDLEY_DIAGNOSTIC_POP
 
 // #include <nlohmann/detail/macro_scope.hpp>
 
@@ -5975,6 +5973,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/abi_macros.hpp>
 
+// #include <nlohmann/detail/macro_scope.hpp>
+
 // #include <nlohmann/detail/meta/type_traits.hpp>
 
 // #include <nlohmann/detail/string_utils.hpp>
@@ -6204,10 +6204,10 @@ NLOHMANN_JSON_NAMESPACE_END
 namespace std
 {
 
+// Fix: https://github.com/nlohmann/json/issues/1401
+JSON_HEDLEY_DIAGNOSTIC_PUSH
 #if defined(__clang__)
-    // Fix: https://github.com/nlohmann/json/issues/1401
-    #pragma clang diagnostic push
-    #pragma clang diagnostic ignored "-Wmismatched-tags"
+    JSON_HEDLEY_PRAGMA(clang diagnostic ignored "-Wmismatched-tags")
 #endif
 template<typename IteratorType>
 class tuple_size<::nlohmann::detail::iteration_proxy_value<IteratorType>> // NOLINT(cert-dcl58-cpp)
@@ -6221,9 +6221,7 @@ class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
                      get<N>(std::declval <
                             ::nlohmann::detail::iteration_proxy_value<IteratorType >> ()));
 };
-#if defined(__clang__)
-    #pragma clang diagnostic pop
-#endif
+JSON_HEDLEY_DIAGNOSTIC_POP
 
 }  // namespace std
 
@@ -18857,9 +18855,9 @@ class binary_writer
 
     void write_compact_float(const number_float_t n, detail::input_format_t format)
     {
+        JSON_HEDLEY_DIAGNOSTIC_PUSH
 #ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wfloat-equal"
+        JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
         if (!std::isfinite(n) || ((static_cast<double>(n) >= static_cast<double>(std::numeric_limits<float>::lowest()) &&
                                    static_cast<double>(n) <= static_cast<double>((std::numeric_limits<float>::max)()) &&
@@ -18877,9 +18875,7 @@ class binary_writer
                                 : get_msgpack_float_prefix(n));
             write_number(n);
         }
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
+        JSON_HEDLEY_DIAGNOSTIC_POP
     }
 
   public:
@@ -20052,9 +20048,9 @@ char* to_chars(char* first, const char* last, FloatType value)
         *first++ = '-';
     }
 
+    JSON_HEDLEY_DIAGNOSTIC_PUSH
 #ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wfloat-equal"
+    JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
     if (value == 0) // +-0
     {
@@ -20064,9 +20060,7 @@ char* to_chars(char* first, const char* last, FloatType value)
         *first++ = '0';
         return first;
     }
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
+    JSON_HEDLEY_DIAGNOSTIC_POP
 
     JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10);
 
@@ -25197,15 +25191,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
     bool operator==(const_reference rhs) const noexcept
     {
+        JSON_HEDLEY_DIAGNOSTIC_PUSH
 #ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wfloat-equal"
+        JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
         const_reference lhs = *this;
         JSON_IMPLEMENT_OPERATOR( ==, true, false, false)
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
+        JSON_HEDLEY_DIAGNOSTIC_POP
     }
 
     /// @brief comparison: equal
@@ -25290,14 +25282,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/operator_eq/
     friend bool operator==(const_reference lhs, const_reference rhs) noexcept
     {
+        JSON_HEDLEY_DIAGNOSTIC_PUSH
 #ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wfloat-equal"
+        JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
         JSON_IMPLEMENT_OPERATOR( ==, true, false, false)
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
+        JSON_HEDLEY_DIAGNOSTIC_POP
     }
 
     /// @brief comparison: equal