Two changes in this checkin. Added a ThreadPlanKind so that I can do some...

Two changes in this checkin.  Added a ThreadPlanKind so that I can do some reasoning based on the kind of thread plan
without having to use RTTI.
Removed the ThreadPlanContinue and replaced with a ShouldAutoContinue query that serves the same purpose.  Having to push
another plan to assert that if there's no other indication the target should continue when this plan is popped was flakey
and error prone.  This method is more stable, and fixed problems we were having with thread specific breakpoints.

llvm-svn: 106378