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