The test harness now can recover after test crashes, allowing:
- check for crashes themselves (without reporting them to the Python lit driver,
which is about 10x slower -- even if CrashTracer is disabled);
- recover from unexpected test crashes and run the rest of the tests;
- this lays the groundwork for assertions that end the test execution, but
allow the rest of the tests to run (rdar://17906801).
Note that we don't spawn a fresh process for every test. We create a child
process and reuse it until it crashes.
Swift SVN r21090
