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>

tests/src/unit-allocator.cpp

@@ -24,7 +24,7 @@ struct bad_allocator : std::allocator<T>
     template<class U> bad_allocator(const bad_allocator<U>& /*unused*/) { }
 
     template<class... Args>
-    void construct(T* /*unused*/, Args&& ... /*unused*/) // NOLINT(cppcoreguidelines-missing-std-forward)
+    [[noreturn]] void construct(T* /*unused*/, Args&& ... /*unused*/) // NOLINT(cppcoreguidelines-missing-std-forward)
     {
         throw std::bad_alloc();
     }