Guard JSON_HEDLEY_DIAGNOSTIC_PUSH/POP with the same compiler check as the pragma they bracket

Addresses review feedback from @gregmarr on PR #5485: the push/pop calls
were unconditional, so compilers other than the one the ignored-pragma
targets (e.g. MSVC, or GCC where the pair only applies under __clang__)
now did a needless push/pop with nothing suppressed in between. Move the
existing #ifdef __GNUC__ / #if defined(__clang__) guard to also cover the
push/pop, restoring the original zero-overhead behavior on other compilers
while still emitting the pragma itself through Hedley.

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

include/nlohmann/json.hpp

@@ -3769,13 +3769,15 @@ 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__
+        JSON_HEDLEY_DIAGNOSTIC_PUSH
         JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
         const_reference lhs = *this;
         JSON_IMPLEMENT_OPERATOR( ==, true, false, false)
+#ifdef __GNUC__
         JSON_HEDLEY_DIAGNOSTIC_POP
+#endif
     }
 
     /// @brief comparison: equal
@@ -3860,12 +3862,14 @@ 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__
+        JSON_HEDLEY_DIAGNOSTIC_PUSH
         JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #endif
         JSON_IMPLEMENT_OPERATOR( ==, true, false, false)
+#ifdef __GNUC__
         JSON_HEDLEY_DIAGNOSTIC_POP
+#endif
     }
 
     /// @brief comparison: equal