[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/unordered_map

@@ -361,10 +361,8 @@ template <class Key, class T, class Hash, class Pred, class Alloc>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Key, class _Cp, class _Hash, bool = is_empty<_Hash>::value
-#if __has_feature(is_final)
-                                         && !__is_final(_Hash)
-#endif
+template <class _Key, class _Cp, class _Hash,
+          bool = is_empty<_Hash>::value && !__libcpp_is_final<_Hash>::value
          >
 class __unordered_map_hasher
     : private _Hash
@@ -412,10 +410,8 @@ public:
         {return __hash_(__x);}
 };
 
-template <class _Key, class _Cp, class _Pred, bool = is_empty<_Pred>::value
-#if __has_feature(is_final)
-                                         && !__is_final(_Pred)
-#endif
+template <class _Key, class _Cp, class _Pred,
+          bool = is_empty<_Pred>::value && !__libcpp_is_final<_Pred>::value
          >
 class __unordered_map_equal
     : private _Pred