[libcxx] Add paranoid cast-to-void in comma operator

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348611 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/memory b/include/memory
index 9fca9fe..3e8f593 100644
--- a/include/memory
+++ b/include/memory
@@ -1613,7 +1613,7 @@
         void
         __construct_forward(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __begin2)
         {
-            for (; __begin1 != __end1; ++__begin1, ++__begin2)
+            for (; __begin1 != __end1; ++__begin1, (void) ++__begin2)
                 construct(__a, _VSTD::__to_raw_pointer(__begin2), _VSTD::move_if_noexcept(*__begin1));
         }