Make sure that when if we are going to Halt while the process is in the middle...

Make sure that when if we are going to Halt while the process is in the middle of HandlePrivateEvent we
wait till that is done.  We need a stronger way to do this, but in practice this works and using some locking
strategy is harder because Halt & HandlePrivateEvent generally happen on different threads.

llvm-svn: 158042