The current machinery using KeyboardInterrupt for canceling doesn't work
with multiple threads on Windows as it just cancels the currently run tests
but the runners continue.
We install a handler for Ctrl-C which stops the provider from providing any
more tests to the runners. Together with aborting all currently running
tests, this brings lit to a halt.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186695 91177308-0d34-0410-b5e6-96231b3b80d8