[libcxx] Fix use of operator comma where the types can be user defined

Summary:
An evil user might overload operator comma. Use a void cast to make sure any user overload is not selected.
Modify all the test iterators to define operator comma. 

Reviewers: danalbert, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5929

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220706 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -2215,7 +2215,7 @@ basic_string<_CharT, _Traits, _Allocator>::__init(_ForwardIterator __first, _For
         __set_long_cap(__cap+1);
         __set_long_size(__sz);
     }
-    for (; __first != __last; ++__first, ++__p)
+    for (; __first != __last; ++__first, (void) ++__p)
         traits_type::assign(*__p, *__first);
     traits_type::assign(*__p, value_type());
 }