Driver: Implement Darwin_X86 tool chain level argument translation.

 - This is really gross, but its the easiest way to match gcc. Once we
   are confident in the driver, we can try and push these translations
   down into tools.

 - No test cases for this yet, it's hard to see the effects of these
   translations before the gcc tool argument translation is pulled
   over.

 - Interaction with "unused argument" warning hasn't been worked out
   yet.

 - <rdar://problem/6717359> [driver] implement toolchain specific
   argument translation.

"It's horrible in here."

llvm-svn: 67683