[lit] Try to exit more cleanly

If all jobs complete successfully, use pool.close() instead of
pool.terminate() before waiting for the workers. Zach Turner reported
that he was getting "access denied" exceptions from pool.terminate().

Make the workers abort immediately without printing to stderr when they
are interrupted.

Finally, catch exceptions when attempting to remove our temporary
testing directory. On abnormal exit, there can often be open handles
that haven't been cleaned up yet.

llvm-svn: 301941

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