[libcxx] Add --show-unsupported and --show-xfail to check-libcxx's default LIT args.

Summary:
In order to gather more information about testsuite results these flags should be added to LIT's default args.
These new switches were recently added to LIT. It been more than two weeks since both switches were added.
I think its time we add these to our LIT flags.


Reviewers: mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5037