[libcxx] Add UNSUPPORTED tag to lit. It mirrors REQUIRES.

Summary:
This patch adds support for // UNSUPPORTED: feature. If an excluded feature is found in the list of available features then the test is marked unsupported.

I hope to use this to mark test unsupported if the fail with msan on asan. As well as tests that fail in particular standard modes.


Reviewers: mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4950