Add __unconstref for future use

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@263659 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/type_traits b/include/type_traits
index 47f9334..2922839 100644
--- a/include/type_traits
+++ b/include/type_traits
@@ -1072,6 +1072,11 @@
     typedef typename remove_cv<typename remove_reference<_Tp>::type>::type type;
 };
 
+template <class _Tp>
+struct __unconstref {
+    typedef typename remove_const<typename remove_reference<_Tp>::type>::type type;
+};
+
 // __is_same_uncvref
 
 template <class _Tp, class _Up>