[lldb] Use weak_ptr to hold on to the underlying thread plan in SBThreadPlan

Use a weak pointer to hold on to the the underlying thread plan in
SBThreadPlan. When the process continues, all the popped ThreadPlans get
discarded, and you can’t reuse them, so you have to create them anew.
Therefore the SBThreadPlan doesn’t need to keep the ThreadPlan alive.

This fixes the cleanup error in TestThreadPlanCommands.py and
TestStepScripted.py caused by the thread plans never being deleted.

Differential revision: https://reviews.llvm.org/D84210