[libcxx] Fix detection of __is_final.

Summary: Currently we only enable the use of __is_final(...) with Clang. GCC also provides __is_final(...) since 4.7 in all standard modes. This patch creates the macro _LIBCPP_HAS_IS_FINAL to note the availability of `__is_final`.

Reviewers: danalbert, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8795

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239664 91177308-0d34-0410-b5e6-96231b3b80d8

include/tuple

@@ -161,10 +161,8 @@ using tuple_element_t = typename tuple_element <_Ip, _Tp...>::type;
 
 // __tuple_leaf
 
-template <size_t _Ip, class _Hp, bool=is_empty<_Hp>::value
-#if __has_feature(is_final)
-                                 && !__is_final(_Hp)
-#endif
+template <size_t _Ip, class _Hp,
+          bool=is_empty<_Hp>::value && !__libcpp_is_final<_Hp>::value
          >
 class __tuple_leaf;