libstdc++: Fix definition of std::remove_cvref_t

libstdc++: Fix definition of std::remove_cvref_t

I originally defined std::remove_cvref_t in terms of the internal
__remove_cvref_t trait, to avoid instantiating the remove_cvref class
template. However, as described in P1715R0 that is observable by users
and is thus non-conforming.

This defines remove_cvref_t as specified in the standard.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (remove_cvref_t): Define in terms of
	remove_cvref.
	* testsuite/20_util/remove_cvref/value.cc: Check alias.