Make reverse() call iter_swap like the standard says, instead of calling swap directly. No real change.

llvm-svn: 251836
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 9088829..9c05119 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -2330,7 +2330,7 @@
     {
         if (__first == --__last)
             break;
-        swap(*__first, *__last);
+        _VSTD::iter_swap(__first, __last);
         ++__first;
     }
 }
@@ -2342,7 +2342,7 @@
 {
     if (__first != __last)
         for (; __first < --__last; ++__first)
-            swap(*__first, *__last);
+            _VSTD::iter_swap(__first, __last);
 }
 
 template <class _BidirectionalIterator>