[lit] Cleanly exit on user keyboard interrupt

Graceful lit shutdown on user keyboard interrupt [Ctrl+C] was a
longstanding goal of mine.  After a few refactorings this revision
finally enables it.  We use the following strategy to deal with
KeyboardInterrupt:
https://noswap.com/blog/python-multiprocessing-keyboardinterrupt

Printing of a helpful summary for interrupted runs (just as the one for
completed runs) will be tackled in future revisions.

Reviewed By: serge-sans-paille, rnk

Differential Revision: https://reviews.llvm.org/D77365