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

llvm-svn: 261278