Fix stale Clang -Weverything suppression comments; drop -Wno-missing-noreturn (#5250)

* Fix stale Clang -Weverything suppression comments; eliminate -Wno-missing-noreturn

cmake/clang_flags.cmake claimed -Wno-unsafe-buffer-usage was needed only
for Doctest and that -Wno-missing-noreturn had "no way to silence...
otherwise" (PR #4871, which never actually attempted a source fix).
Neither held up under investigation (todo 130):

- -Wno-unsafe-buffer-usage is pervasive (208 distinct sites across 19
  files measured with clang trunk in silkeh/clang:dev), spanning the
  library's own low-level numeric/buffer code (to_chars, serializer,
  lexer, binary reader/writer, input adapters, json_pointer) as well as
  vendored Doctest itself (96 of the 208 sites). A source-level fix is
  not feasible at this scale; the comment now says so instead of
  blaming Doctest alone.

- -Wno-missing-noreturn had exactly two real trigger sites, both
  genuinely and unconditionally non-returning: a test-only throwing
  allocator (tests/src/unit-allocator.cpp) and, previously undiscovered,
  wide_string_input_adapter::get_elements<T>() in
  include/nlohmann/detail/input/input_adapters.hpp. Verified this isn't
  a wider pattern by checking all 160 JSON_THROW call sites in the
  library for functions whose entire body is an unconditional throw.
  Annotated both ([[noreturn]] in the test file, since JSON_HEDLEY_NO_RETURN
  is #undef'd by the time test code runs; JSON_HEDLEY_NO_RETURN in the
  library file, its first real use anywhere in the codebase) and
  dropped the suppression entirely.

single_include/nlohmann/json.hpp regenerated via `make amalgamate`;
`make check-amalgamation` passes.

Verified in Docker (silkeh/clang:dev, matching the ci_static_analysis_clang
CI job): baseline builds clean, and the full 194-target test suite builds
with zero warnings under the corrected CLANG_CXXFLAGS (-Wno-missing-noreturn
no longer in the list). Also sanity-compiled and ran unit-allocator.cpp and
unit-wstring.cpp on host Apple Clang to confirm behavior is unchanged.

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

* Fix MSVC C4702 warning caused by JSON_HEDLEY_NO_RETURN on get_elements()

PR #5250 annotated wide_string_input_adapter::get_elements<T>() with
JSON_HEDLEY_NO_RETURN (it unconditionally throws). On MSVC this expands to
__declspec(noreturn), and MSVC correctly determined that the code following
its call in binary_reader.hpp is unreachable for that instantiation, firing
C4702 under /W4 /WX in the msvc, msvc-vs2026, and msvc-arm64 Debug jobs.

Clang doesn't flag this case, so the Docker verification for #5250 (which
only checked Clang -Weverything) didn't catch it.

This is the same warning class already tolerated for Release builds since
PR #5216, where MSVC's optimizer independently found the same dead code
after /Od was removed. Extend that existing /wd4702 suppression to Debug
builds too, instead of reverting the noreturn annotation.

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

---------

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

include/nlohmann/detail/input/input_adapters.hpp

@@ -430,7 +430,7 @@ class wide_string_input_adapter
 
     // parsing binary with wchar doesn't make sense, but since the parsing mode can be runtime, we need something here
     template<class T>
-    std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
+    JSON_HEDLEY_NO_RETURN std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
     {
         JSON_THROW(parse_error::create(112, 1, "wide string type cannot be interpreted as binary data", nullptr));
     }