If the ObjC Step Through Trampoline plan causes a target crash, properly...

If the ObjC Step Through Trampoline plan causes a target crash, properly propagate the error back to
the controlling plans so that they don't lose control.

Also change "ThreadPlanStepThrough" to take the return StackID for its backstop breakpoint as an argument
to the constructor rather than having it try to figure it out itself, since it might get it wrong whereas
the caller always knows where it is coming from.

rdar://problem/11402287

llvm-svn: 156529