Minor bug fix for allowing an extension of const-qualified types in containers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@183481 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/memory b/include/memory
index 912a43f..bcf4d6d 100644
--- a/include/memory
+++ b/include/memory
@@ -1750,7 +1750,7 @@
     typedef const _Tp*        const_pointer;
     typedef const _Tp&        reference;
     typedef const _Tp&        const_reference;
-    typedef _Tp               value_type;
+    typedef const _Tp         value_type;
 
     typedef true_type propagate_on_container_move_assignment;