thread state coordinator: added new thread support.

A new thread arriving while a pending signal notification
is outstanding will (1) add the new thread to the list of
stops expected before the deferred signal notification is
fired, (2) send a stop request for the new thread, and
(3) track the new thread as currently running.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218578 91177308-0d34-0410-b5e6-96231b3b80d8