libstdc++: Make std::variant work with Clang in C++20 mode [PR103891]

libstdc++: Make std::variant work with Clang in C++20 mode [PR103891]

Clang has some bugs with destructors that use constraints to be
conditionally trivial, so disable the P2231R1 constexpr changes to
std::variant unless the compiler is GCC 12 or later.

If/when P2493R0 gets accepted and implemented by G++ we can remove the
__GNUC__ check and use __cpp_concepts >= 202002 instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/103891
	* include/bits/c++config (_GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS):
	Define.
	* include/std/variant (__cpp_lib_variant): Only define C++20
	value when the compiler is known to support conditionally
	trivial destructors.
	* include/std/version (__cpp_lib_variant): Likewise.