libstdc++: Fix fallback definitions of std::is_member_*_pointer

libstdc++: Fix fallback definitions of std::is_member_*_pointer

When the builtins aren't used we need a declaration of std::is_function
for the fallback definitions of std::is_member_function_pointer and
std::is_member_object_pointer.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_function): Declare before first
	use.