Release CordbUnmanagedThread instance from CordbProcess member

[AaronRobinsonMSFT]

Fixes #65357

The m_lastDispatchedIBEvent was not being cleared during shutdown, which was causing a memory leak assert to fire.

See

runtime/src/coreclr/debug/di/process.cpp

Lines 8130 to 8148 in c3fa765

	// Make sure we keep the thread alive while we're playing with it.
	pUnmanagedThread->InternalAddRef();
	LOG((LF_CORDB, LL_INFO10000, "CP::DUE: dispatching unmanaged event %d for thread 0x%x\n",
	pUnmanagedEvent->m_currentDebugEvent.dwDebugEventCode, pUnmanagedThread->m_id));
	m_dispatchingUnmanagedEvent = true;
	// Add/Remove a reference which is scoped to the time that m_lastDispatchedIBEvent
	// is set to pUnmanagedEvent (it is an interior pointer)
	// see DevDiv issue 818301 for more details
	if(m_lastDispatchedIBEvent != NULL)
	{
	m_lastDispatchedIBEvent->m_owner->InternalRelease();
	m_lastDispatchedIBEvent = NULL;
	}
	pUnmanagedThread->InternalAddRef();
	m_lastDispatchedIBEvent = pUnmanagedEvent;
	pUnmanagedEvent->SetState(CUES_Dispatched);

I added the clearing in the NeuterChildren() method because it seemed like the best place. Suggestions on more appropriate places is welcome.

/cc @dotnet/dotnet-diag

[AaronRobinsonMSFT]

/cc @noahfalk @hoyosjs

[hoyosjs]

Feels pretty safe - this is a flush, shutdown, or detach path. Might not mimic the best lifetime for the event being held but can't think of a better place to have it - you need it for other continue events and native event convoys. I couldn't find a case that wouldn't fall in this scenario as long as the debugger impl is checking the shutdown/detach path for interop.