libstdc++: Add std::bit_cast for C++20 [PR 93121]

libstdc++: Add std::bit_cast for C++20 [PR 93121]

Thanks to Jakub's addition of the built-in, we can add this to the
library now. The compiler tests for the built-in are quite extensive,
including verifying the constraints, so this only adds minimal tests to
the library testsuite.

This doesn't add a new _GLIBCXX_HAVE_BUILTIN_BIT_CAST because using
__has_builtin(__builtin_bit_cast) works for GCC and versions of Clang
that provide the built-in.

libstdc++-v3/ChangeLog:

	PR libstdc++/93121
	* include/std/bit (__cpp_lib_bit_cast, bit_cast): Define.
	* include/std/version (__cpp_lib_bit_cast): Define.
	* testsuite/26_numerics/bit/bit.cast/bit_cast.cc: New test.
	* testsuite/26_numerics/bit/bit.cast/version.cc: New test.