Fix a misunderstanding of the ThreadPlan::OkayToDiscard flag in InferiorCallPOSIX. It was

set to true, but all plans run by RunThreadPlan need to have this set to false so they will
return control to RunThreadPlan without consulting plans higher on the stack.

Since this seems like a common error, I also modified RunThreadPlan to enforce this behavior.

<rdar://problem/22543166>

llvm-svn: 250084