libstdc++: Use __is_same instead of __is_same_as

libstdc++: Use __is_same instead of __is_same_as

PR 92271 added __is_same as another spelling of __is_same_as. Since
Clang also spells it __is_same, let's just use that consistently.

It appears that Intel icc sets __GNUC__ to 10, but only supports
__is_same_as. If we only use __is_same for __GNUC__ >= 11 then we won't
break icc again (it looks like we broke previous versions of icc when we
started using __is_same_as).

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_HAVE_BUILTIN_IS_SAME):
	Define for GCC 11 or when !__is_identifier(__is_same).
	(_GLIBCXX_BUILTIN_IS_SAME_AS): Remove.
	* include/std/type_traits (is_same, is_same_v): Replace uses
	of _GLIBCXX_BUILTIN_IS_SAME_AS.