SBProcess::Kill can crash the debugger

[labath]

SBProcess::Kill calls ThreadPlanStack::DiscardAllPlans. If someone calls this while the process is in getting ready to stop (for example, because it has just completed a thread plan), then the internal state thread will hit the assert in ThreadPlanStack::PopPlan because there is nothing to pop.

This happened in TestGlobalModuleCache.test_OneTargetTwoDebuggers due to a bad test, but it can happen in real uses as well.

[llvmbot]

@llvm/issue-subscribers-lldb

Author: Pavel Labath (labath)

`SBProcess::Kill` calls `ThreadPlanStack::DiscardAllPlans`. If someone calls this while the process is in getting ready to stop (for example, because it has just completed a thread plan), then the internal state thread will hit the assert in `ThreadPlanStack::PopPlan` because there is nothing to pop.

This happened in TestGlobalModuleCache.test_OneTargetTwoDebuggers due to a bad test, but it can happen in real uses as well.

[labath]

Actually, never mind, @DavidSpickett has a more detailed explanation of the problem in #76057.