Add a new watchdog timer interface. The interface does not permit handling timeouts, so
it's only really useful if you're going to crash anyways. Use it in the pretty stack trace
printer to kill the compiler if we hang while printing the stack trace.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177962 91177308-0d34-0410-b5e6-96231b3b80d8