SFINAE __bit_iterator such that it will only get instantiated with a container that has the nested type __storage_type.  This prevents accidental instantiation such as in http://llvm.org/bugs/show_bug.cgi?id=12755.  This fixes http://llvm.org/bugs/show_bug.cgi?id=12755.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@156308 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/vector b/include/vector
index 61f0aef..0d5b6b8 100644
--- a/include/vector
+++ b/include/vector
@@ -1970,6 +1970,7 @@
     typedef allocator_traits<allocator_type>         __alloc_traits;
     typedef typename __alloc_traits::size_type       size_type;
     typedef typename __alloc_traits::difference_type difference_type;
+    typedef size_type __storage_type;
     typedef __bit_iterator<vector, false>            pointer;
     typedef __bit_iterator<vector, true>             const_pointer;
 #ifdef _LIBCPP_DEBUG
@@ -1991,7 +1992,6 @@
     typedef _VSTD::reverse_iterator<const_iterator>   const_reverse_iterator;
 
 private:
-    typedef size_type __storage_type;
     typedef typename __alloc_traits::template
 #ifndef _LIBCPP_HAS_NO_TEMPLATE_ALIASES
                 rebind_alloc<__storage_type>