Finish implementing "Treating Unnecessary decay" (P0777R1)

Finish implementing "Treating Unnecessary decay" (P0777R1)

	* include/std/tuple (apply): Use remove_reference_t instead of decay_t
	as per P0777R1.
	* include/std/type_traits (__result_of_memfun): Use remove_reference
	instead of __remove_cvref_t and remove redundant is_same check.
	(__inv_unwrap): Use __remove_cvref_t instead of decay_t.

From-SVN: r270551