[driver] Add the missing TY_PP_ObjCXX_Alias case to the isCXX function.

This was causing different behavior when using -x objective-c++-cpp-output as
compared to -x objc++-cpp-output.  Specifically, the driver was not adding the
-fcxx-exceptions flag in the latter case.
rdar://12680312

llvm-svn: 168212