libstdc++: use __bool_constant instead of integral_constant

libstdc++: use __bool_constant instead of integral_constant

In the type_traits header, both integral_constant<bool> and __bool_constant
are used. This patch unifies those usages into __bool_constant.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Use __bool_constant instead of
	integral_constant.

Signed-off-by: Ken Matsui <kmatsui@cs.washington.edu>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>