[driver] Warnings for warning options are handled by the frontend.  The driver needs to process the
warning options to setup diagnostic state, but should not be emitting warnings as these would be
rudndant with what the frontend emits.
rdar://13001556

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172497 91177308-0d34-0410-b5e6-96231b3b80d8