[PATCH] [libcxx] Refactor lit.cfg.

[Eric Fiselier]

It makes me very happy to approve this! Now commit it before we have merge conflicts!

A few nits you can choose to ignore or implement:
- I had trouble finding the start to the configuration class. Maybe add better visual separation.
- Perhaps we can add better visual separation between methods as well.
- We lose a lot of space to indentation. I would prefer 2 space tabs. (even though all my editors are set to 4).

Either way LGTM.

http://reviews.llvm.org/D4952