Don't warn about -funit-at-a-time, and reject -fno-unit-at-a-time.

 - We could just warn about -fno-unit-at-a-time, but in practice people using it
   probably aren't going to get what they want out of clang.

Also, use "clang" specified error for unsupported things instead of driver
unsupported error.

llvm-svn: 72272