Add a test case that would have caught the bug fixed in r126640.

I've only covered C++'s flags here, we should probably do something
similar for ObjC flags.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126641 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed