Use __is_same_as for std::is_same and std::is_same_v

Use __is_same_as for std::is_same and std::is_same_v

By using the built-in we don't need to match a partial specialization
for std::is_same and don't need to instantiate std::is_same at all for
uses of std::is_same_v.

	* include/std/type_traits (is_same): Replace partial specialization
	by using __is_same_as built-in in primary template.
	(is_same_v): Use __is_same_as built-in instead of instantiating the
	is_same trait.

From-SVN: r276891