Two changes,

1) Start the PrivateStateThread stopped, and then in
StartPrivateStateThread, make the private state thread and then
resume it before we say the thread is created.  That way we know it is
listening for events by the time we get out of
StartPrivateStateThread.

2) Backstop running a thread plan when calling Process::RunThreadPlan
on the private state thread with a ThreadPlanBase so that running the
plan doesn't pass its stop events to whatever plans happen to be above
us on the thread plan stack.

llvm-svn: 154368