lit: Support cancellation on Windows

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