[PATCH] [Driver] Refactor clang driver to use LLVM's Option library

[Rafael Espíndola]

> I could reduce the diff significantly with using directives in the
> internal Driver and Frontend header files if people feel like that's OK.

I really like this patch!

Would it be too much work to do it in two patches? The first one would
include the using directive and be fairly easy to read. The second one
would be a completely mechanical removal of those directives.

Thanks,
Rafael