[libcxx] Add compiler utility class for LIT tests

Summary:
This adds a compiler utility class that handles generating compile commands and running them. Instead of having to pass the compiler path and 3 groups of flags between the configuration and the test format this allows us to just pass the compiler object.



Reviewers: danalbert, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7019

llvm-svn: 226323