[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