[ORC] Shut down dispatcher in ExecutorProcessControl implementations.

f341161 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

llvm/lib/ExecutionEngine/Orc/ExecutorProcessControl.cpp

@@ -130,7 +130,10 @@ void SelfExecutorProcessControl::callWrapperAsync(ExecutorAddr WrapperFnAddr,
   SendResult(WrapperFn(ArgBuffer.data(), ArgBuffer.size()));
 }
 
-Error SelfExecutorProcessControl::disconnect() { return Error::success(); }
+Error SelfExecutorProcessControl::disconnect() {
+  D->shutdown();
+  return Error::success();
+}
 
 void SelfExecutorProcessControl::writeUInt8sAsync(
     ArrayRef<tpctypes::UInt8Write> Ws, WriteResultFn OnWriteComplete) {

llvm/lib/ExecutionEngine/Orc/SimpleRemoteEPC.cpp

@@ -92,6 +92,7 @@ void SimpleRemoteEPC::callWrapperAsync(ExecutorAddr WrapperFnAddr,
 
 Error SimpleRemoteEPC::disconnect() {
   T->disconnect();
+  D->shutdown();
   std::unique_lock<std::mutex> Lock(SimpleRemoteEPCMutex);
   DisconnectCV.wait(Lock, [this] { return Disconnected; });
   return std::move(DisconnectErr);