Make the "synchronous" mode actually work without race conditions.

Make the "synchronous" mode actually work without race conditions.

There were many issues with synchronous mode that we discovered when started to try and add a "batch" mode. There was a race condition where the event handling thread might consume events when in sync mode and other times the Process::WaitForProcessToStop() would consume them. This also led to places where the Process IO handler might or might not get popped when it needed to be.

llvm-svn: 220254