Prevent recursive instantiation in std::function

Prevent recursive instantiation in std::function

	PR libstdc++/69005
	PR libstdc++/69222
	* include/std/functional (function::_Invoke): Remove, use result_of.
	(function::_Callable): Replace alias template with class template
	and use partial specialization instead of _NotSelf alias template.
	(function(_Functor)): Add "not self" constraint so that _Callable is
	not used while type is incomplete.
	* testsuite/20_util/function/69222.cc: New.

From-SVN: r232273