Implement C++17 std::invoke and LWG DR 2219

Implement C++17 std::invoke and LWG DR 2219

	* include/std/functional (__invoke_impl): New overloads.
	(__invoke): Replace with a single function calling __invoke_impl.
	(invoke): Add C++17 std::invoke.
	(reference_wrapper::operator()): Qualify call to __invoke.
	(_Mem_fn_traits_base, _Mem_fn_traits): Remove unused typedefs.
	(_Mem_fn_base): Remove unused typedefs and implement call operator in
	terms of __invoke.
	* include/std/future (__future_base::_Async_state_commonV2): Do not
	pass reference_wrapper as object argument to call_once.
	* include/std/type_traits (result_of): Define nested __invoke_type.
	Handle reference_wrapper as per LWG 2219.
	* testsuite/20_util/bind/ref_neg.cc: Adjust dg-error directives.
	* testsuite/20_util/function_objects/mem_fn/55463.cc: Remove tests
	using member functions of reference_wrapper.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@229290 138bc75d-0d04-0410-961f-82ee72b054a4