libstdc++: Diagnose visitors with different return types [PR95904]

libstdc++: Diagnose visitors with different return types [PR95904]

libstdc++-v3/ChangeLog:

	PR libstdc++/95904
	* include/std/variant (__deduce_visit_result): Add a nested ::type.
	(__gen_vtable_impl</*base case*/>::_S_apply):
	Check the visitor return type.
	(__same_types): New.
	(__check_visitor_result): Likewise.
	(__check_visitor_results): Likewise.
	(visit(_Visitor&&, _Variants&&...)): Use __check_visitor_results
	in case we're visiting just one variant.
	* testsuite/20_util/variant/visit_neg.cc: Adjust.