[lldb/telemetry] Report exit status only once (#134078)
SetExitStatus can be called the second time when we reap the debug
server process. This shouldn't be interesting as at that point, we've
already told everyone that the process has exited.
I believe/hope this will also help with sporadic shutdown crashes that
have cropped up recently. They happen because the debug server is
monitored from a detached thread, so this code can be called after main
returns (and starts destroying everything). This isn't a real fix for
that though, as the situation can still happen (it's just that it
usually happens after the exit status has already been set). I think the
real fix for that is to make sure these threads terminate before we
start shutting everything down.
