[LIT] Move display of unsupported and xfail tests to summary.

Summary:
This patch changes the way xfail and unsupported tests are displayed.
This output is only displayed when the --show-unsupported/--show-xfail flags are passed to lit.

Currently xfail/unsupported tests are printed during the run of the test-suite. I think its better to display this information during the summary instead.
This patch removes the printing of these tests from when they are run to the summary.


Reviewers: ddunbar, EricWF

Reviewed By: EricWF

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4842