libstdc++: Optimize std::is_function compilation performance

libstdc++: Optimize std::is_function compilation performance

This patch optimizes the compilation performance of std::is_function
by dispatching to the new __is_function built-in trait.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_function): Use __is_function
	built-in trait.
	(is_function_v): Likewise. Optimize its implementation.  Move
	this under is_const_v as this depends on is_const_v.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>