libstdc++: Use __is_invocable/nothrow_invocable builtins more

libstdc++: Use __is_invocable/nothrow_invocable builtins more

As a follow-up to r15-1253 and r15-1254 which made us use these builtins
in the standard std::is_invocable/nothrow_invocable class templates, let's
also use them directly in the standard variable templates and our internal
C++11 __is_invocable/nothrow_invocable class templates.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__is_invocable): Define in terms of
	corresponding builtin if available.
	(__is_nothrow_invocable): Likewise.
	(is_invocable_v): Likewise.
	(is_nothrow_invocable_v): Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>