libstdc++: Optimize std::is_compound compilation performance

This patch optimizes the compilation performance of std::is_compound.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_compound): Do not use __not_.
	(is_compound_v): Use is_fundamental_v instead.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>