Add -Wcomma warning to Clang.

Authored by rtrieu on Feb 18 2016, 3:58 PM.


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