Clang doesn't support __is_same_as but provides __is_same instead.
Restore the original implementation (pre r276891) when neither of those
built-ins is available.
* include/bits/c++config (_GLIBCXX_BUILTIN_IS_SAME_AS): Define to
one of __is_same_as or __is_same when available.
* include/std/concepts (__detail::__same_as): Use std::is_same_v.
* include/std/type_traits (is_same) [_GLIBCXX_BUILTIN_IS_SAME_AS]:
Use new macro instead of __is_same_as.
(is_same) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Restore partial
specialization.
(is_same_v) [_GLIBCXX_BUILTIN_IS_SAME_AS]: Use new macro.
(is_same_v) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Use std::is_same.
From-SVN: r277058
