do not define operator!= in C++20

A change to the semantics of equality operator rewriting in C++20 (P2468R2: The Equality Operator You Are Looking For) means that operator== may not be rewritten with reversed operands if operator!= is also defined. Since operator!= can normally be synthesized from operator== regardless in this language standard, we can and should avoid defining those when the new language semantics are available.

This fixes the compilation of tests (and probably consuming code) in C++20 onwards for compilers that implement this new semantic, including recent nightly builds of clang-16.

Reference: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2468r2.html

include/rapidjson/document.h

@@ -1078,6 +1078,7 @@ public:
     */
     template <typename T> RAPIDJSON_DISABLEIF_RETURN((internal::OrExpr<internal::IsPointer<T>,internal::IsGenericValue<T> >), (bool)) operator==(const T& rhs) const { return *this == GenericValue(rhs); }
 
+#ifndef __cpp_impl_three_way_comparison
     //! Not-equal-to operator
     /*! \return !(*this == rhs)
      */
@@ -1092,7 +1093,6 @@ public:
      */
     template <typename T> RAPIDJSON_DISABLEIF_RETURN((internal::IsGenericValue<T>), (bool)) operator!=(const T& rhs) const { return !(*this == rhs); }
 
-#ifndef __cpp_impl_three_way_comparison
     //! Equal-to operator with arbitrary types (symmetric version)
     /*! \return (rhs == lhs)
      */