Avoid stalls when MainLoop::Interrupt fails to wake up the MainLoop (#164905)

Turns out there's a bug in the current lldb sources that if you fork,
set the stdio file handles to close on exec and then exec lldb with some
commands and the `--batch` flag, lldb will stall on exit. The first
cause of the bug is that the Python session handler - and probably other
places in lldb - think 0, 1, and 2 HAVE TO BE the stdio file handles,
and open and close and dup them as needed. NB: I am NOT trying to fix
that bug. I'm not convinced running the lldb driver headless is worth a
lot of effort, it's just as easy to redirect them to /dev/null, which
does work.

But I would like to keep lldb from stalling on the way out when this
happens. The reason we stall is that we have a MainLoop waiting for
signals, and we try to Interrupt it, but because stdio was closed, the
interrupt pipe for the MainLoop gets the file descriptor 0, which gets
closed by the Python session handler if you run some script command. So
the Interrupt fails.

We were running the Write to the interrupt pipe wrapped in
`llvm::cantFail`, but in a no asserts build that just drops the error on
the floor. So then lldb went on to call std::thread::join on the still
active MainLoop, and that stalls

I made Interrupt (and AddCallback & AddPendingCallback) return a bool
for "interrupt success" instead. All the places where code was
requesting termination, I added checks for that failure, and skip the
std::thread::join call on the MainLoop thread, since that is almost
certainly going to stall at this point.

I didn't do the same for the Windows MainLoop, as I don't know if/when
the WSASetEvent call can fail, so I always return true here. I also
didn't turn the test off for Windows. According to the Python docs all
the API's I used should work on Windows... If that turns out not to be
true I'll make the test Darwin/Unix only.

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