Define std::remove_cvref and std::remove_cvref_t for C++2a

Define std::remove_cvref and std::remove_cvref_t for C++2a

Also define __remove_cvref_t for internal use before C++2a.

	* include/std/any (any_cast): Use __remove_cvref_t.
	* include/std/tuple (__make_tuple): Likewise.
	* include/std/type_traits (__remove_cvref_t): Define.
	(__result_of_memobj, __result_of_memfun): Use __remove_cvref_t.
	[__cplusplus > 201703L] (remove_cvref, remove_cvref_t): Define.
	* include/std/variant (__erased_hash): Use __remove_cvref_t.

From-SVN: r259887