Refactor lit.cfg.

The purely imperative format of the configuration performed in lit.cfg
was making merge conflicts with changes I have for Android an
unbelievable pain in the ass. I've moved all of the configuration into a
Configuration class, with each piece of configuration happening in a
different method. This way I can avoid merge conflicts, and any new
features that get added (as have been done with the sanitizers, the -std
flag, etc.) can be easily applied to Android as well.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Differential Revision: http://reviews.llvm.org/D4952