[lldb/Target] Avoid race between Communication::Disconnect calls.

Avoid a race between the Disconnect call in `Communication::ReadThread`
and the one in `Process::ShouldBroadcastEvent` by reordering the calls
to Disconnect and StopReadThread in `Process::ShouldBroadcastEvent`.

In D77295 Pavel suggested that that might explain the broken pipe I was
seeing. Indeed, changing the order resolved the issue.