[ORC] Shut down dispatcher in ExecutorProcessControl implementations.

f3411616 added a task dispatcher for async handlers, but didn't add a
TaskDispatcher::shutdown call to SelfExecutorProcessControl or SimpleRemoteEPC.
This patch adds the missing call, which ensures that we don't destroy the
dispatcher while tasks are still running.

This should fix the use-after-free crash seen in
https://lab.llvm.org/buildbot/#/builders/5/builds/13063