[libcxx] Tired of colorless compile errors? Enable color diagnostics today!

Summary:
This patch adds a lit option to enable color diagnostics when either `--param=color_diagnostics` is passed to LIT or `LIBCXX_COLOR_DIAGNOSTICS` is present in the environment.

My only concern with this patch is that GCC and Clang take different flags and that only GCC 4.9 and greater support `-fdiagnostics-color=always`

Does anybody have objections to this going in?

Reviewers: jroelofs, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7729

llvm-svn: 229707