[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

test/containers/sequences/forwardlist/forwardlist.cons/assign_range.pass.cpp

@@ -70,7 +70,7 @@ int main()
         typedef input_iterator<const T*> I;
         c.assign(I(std::begin(t0)), I(std::end(t0)));
         int n = 0;
-        for (C::const_iterator i = c.cbegin(); i != c.cend(); ++i, ++n)
+        for (C::const_iterator i = c.cbegin(); i != c.cend(); ++i, (void) ++n)
             assert(*i == 10+n);
         assert(n == 4);
     }