libstdc++: Use _GLIBCXX_USE_BUILTIN_TRAIT for is_same

Since is_same has a fallback native implementation, and
_GLIBCXX_HAVE_BUILTIN_IS_SAME does not support toggling which
implementation to use, we remove the _GLIBCXX_HAVE_BUILTIN_IS_SAME
definition and use _GLIBCXX_USE_BUILTIN_TRAIT instead.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_HAVE_BUILTIN_IS_SAME):
	Removed.
	* include/std/type_traits (is_same): Use
	_GLIBCXX_USE_BUILTIN_TRAIT instead of
	_GLIBCXX_HAVE_BUILTIN_IS_SAME.
	(is_same_v): Likewise.

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