[lldb] Fix PushPlan to set subplan to private

[kastiglione]

Summary

Call SetPrivate(true) for subplans pushed via PushPlan(), as described in its
docstring.

[kastiglione]

Jim previously approved this offline.

lldb/include/lldb/Target/ThreadPlan.h

protected:
 
   virtual bool DoPlanExplainsStop(Event *event_ptr) = 0;
 
   // This pushes a plan onto the plan stack of the current plan's thread.
   // Also sets the plans to private and not master plans.  A plan pushed by
   // another thread plan is never either of the above.
   void PushPlan(lldb::ThreadPlanSP &thread_plan_sp) {
     GetThread().PushPlan(thread_plan_sp);
-    thread_plan_sp->SetPrivate(false);
+    thread_plan_sp->SetPrivate(true);
     thread_plan_sp->SetIsMasterPlan(false);
   }
 
   // This gets the previous plan to the current plan (for forwarding requests).
   // This is mostly a formal requirement, it allows us to make the Thread's
   // GetPreviousPlan protected, but only friend ThreadPlan to thread.
 
   ThreadPlan *GetPreviousPlan() { return GetThread().GetPreviousPlan(this); }