PR libstdc++/59768
	* include/std/functional (_Unwrap, __invfwd): Define.
	(__invoke_impl): Remove reference_wrapper overloads and use __invfwd.
	* include/std/type_traits (__result_of_memobj, __result_of_memfun):
	Add partial specializations for const reference_wrappers and simplify.
	* testsuite/20_util/bind/ref_neg.cc: Use dg-excess-errors.
	* testsuite/20_util/function_objects/invoke/59768.cc: New.

From-SVN: r231574