[lit] Add --show-xfail flag to LIT.

Summary:
This patch add a --show-xfail flag. If this flag is specified then each xfail test will be printed to output.
When it is not given xfail tests are ignored. Ignoring xfail tests is the current behavior.

This flag is meant to mirror the --show-unsupported flag that was recently added.

Reviewers: ddunbar, EricWF

Reviewed By: EricWF

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4750