Allow the ThreadPlanStackMap to hold the thread plans for threads that were not reported by the OS plugin. To facilitate this, move adding/updating the ThreadPlans for a Thread to the ThreadPlanStackMap. Also move dumping thread plans there as well.
Added some tests for "thread plan list" and "thread plan discard" since
I didn't seem to have written any originally.
Differential Revision: https://reviews.llvm.org/D76814