Make the ThreadPlanStepThrough set a backstop breakpoint on the return address from

the function it is being asked to step through, so that even if we get the trampoline
target wrong (for instance) we will still not lose control.

The other fix here is to tighten up the handling of the case where the current plan
doesn't explain the stop, but a plan above us does.  In that case, if the plan that
does explain the stop says it is done, we need to clean up the plans below it and 
continue on with our processing.

llvm-svn: 145740