swift234

Add support for the "leaks" feature to lit.

"leaks" enables the leaks feature if xcrun can find leaks. This only
causes tests that require leaks to run. It does not cause all tests to
run under the leak detector.

In a forthcoming commit I am going to add support for the feature
"leaks-all" which runs leaks on all interpreter tests.

rdar://17083962

Swift SVN r18683