Add -Wcomma warning to Clang.

-Wcomma will detect and warn on most uses of the builtin comma operator.  It
currently whitelists the first and third statements of the for-loop.  For other
cases, the warning can be silenced by casting the first operand of the comma
operator to void.

Differential Revision: http://reviews.llvm.org/D3976


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261278 91177308-0d34-0410-b5e6-96231b3b80d8