gate definition of symmetric equality operators on impl, not lib

These operators call themselves recursively if C++20 semantics are present in the compiler, regardless of standard library support for the operator; therefore the test should be on __cpp_impl_three_way_comparison, not __cpp_lib_[...].

This fixes the Value.EqualtoOperator test when the language standard is set to C++20 and the standard library does not yet define the library support macro.

Author: mumbleskates

include/rapidjson/document.h

@@ -1092,7 +1092,7 @@ class GenericValue {
      */
     template <typename T> RAPIDJSON_DISABLEIF_RETURN((internal::IsGenericValue<T>), (bool)) operator!=(const T& rhs) const { return !(*this == rhs); }
 
-#ifndef __cpp_lib_three_way_comparison
+#ifndef __cpp_impl_three_way_comparison
     //! Equal-to operator with arbitrary types (symmetric version)
     /*! \return (rhs == lhs)
      */