Clang: Stop sending data to backend on disconnect

This addresses

    SOFT ASSERT made fatal: "m_connection.isConnected()" in file
      src/plugins/clangcodemodel/clangbackendipcintegration.cpp, line 230

I could not reproduce the issue locally, so I'm leaving the soft asserts
untouched for now.

Change-Id: If1d55ba7bc7e2d1ac20ad992c6d0d43ceb0f5d73
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/clangbackendipc/connectionclient.cpp

@@ -51,6 +51,7 @@ ConnectionClient::ConnectionClient()
 
     connectLocalSocketError();
     connectLocalSocketConnected();
+    connectLocalSocketDisconnected();
 }
 
 void ConnectionClient::startProcessAndConnectToServerAsynchronously()
@@ -230,6 +231,14 @@ void ConnectionClient::connectLocalSocketConnected()
             &ConnectionClient::resetProcessIsStarting);
 }
 
+void ConnectionClient::connectLocalSocketDisconnected()
+{
+    connect(&localSocket,
+            &QLocalSocket::disconnected,
+            this,
+            &ConnectionClient::disconnectedFromLocalSocket);
+}
+
 void ConnectionClient::finishProcess()
 {
     finishProcess(std::move(process_));