[lldb] Assert immediately prior to calling PopPlan

This is part of ongoing attempts to catch the test from
2684281 failing on Arm and AArch64.

I did get logs for the failure but only on Arm, where the backtrace is
truncated. So, let's do the assert that PopPlan was going to do,
before we call it.

Then I should know exactly which PopPlan is asserting.

Technically I should take a mutex here, but technically I shouldn't
be debugging via buildbot, so I'm going to take the risk temporarily.

Author: DavidSpickett

lldb/source/Target/Thread.cpp

@@ -717,6 +717,16 @@ void Thread::DidResume() {
 
 void Thread::DidStop() { SetState(eStateStopped); }
 
+#define CHECK_BEFORE_POP_PLAN                                                  \
+  {                                                                            \
+    uint32_t i = 0;                                                            \
+    ThreadPlanSP p;                                                            \
+    while ((p = GetPlans().GetPlanByIndex(i, false)))                          \
+      i++;                                                                     \
+    (void)i;
+assert(i != 1 && "Cannot pop plan when there is only one plan (the base plan)");
+}
+
 bool Thread::ShouldStop(Event *event_ptr) {
   ThreadPlan *current_plan = GetCurrentPlan();
 
@@ -831,7 +841,7 @@ bool Thread::ShouldStop(Event *event_ptr) {
             do {
               if (should_stop)
                 current_plan->WillStop();
-              assert(!current_plan->IsBasePlan() && "Cannot pop base plan!");
+              CHECK_BEFORE_POP_PLAN;
               PopPlan();
             } while ((current_plan = GetCurrentPlan()) != prev_plan_ptr);
             // Now, if the responsible plan was not "Okay to discard" then
@@ -884,6 +894,7 @@ bool Thread::ShouldStop(Event *event_ptr) {
           // If a Controlling Plan wants to stop, we let it. Otherwise, see if
           // the plan's parent wants to stop.
 
+          CHECK_BEFORE_POP_PLAN;
           PopPlan();
           if (should_stop && current_plan->IsControllingPlan() &&
               !current_plan->OkayToDiscard()) {
@@ -932,6 +943,7 @@ bool Thread::ShouldStop(Event *event_ptr) {
           // plan is complete but does not explain the stop (example: step to a
           // line with breakpoint), let us move the plan to
           // completed_plan_stack anyway
+          CHECK_BEFORE_POP_PLAN;
           PopPlan();
         } else
           DiscardPlan();