libstdc++: Add std::is_scoped_enum for C++23

libstdc++: Add std::is_scoped_enum for C++23

Implement this C++23 feature, as proposed by P1048R1.

This implementation assumes that a C++23 compiler supports concepts
already. I don't see any point in using preprocessor hacks to detect
compilers which define __cplusplus to a post-C++20 value but don't
support concepts yet.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_scoped_enum): Define.
	* include/std/version (__cpp_lib_is_scoped_enum): Define.
	* testsuite/20_util/is_scoped_enum/value.cc: New test.
	* testsuite/20_util/is_scoped_enum/version.cc: New test.