Clang: Avoid heap-use-after-free in IpcCommunicator

...on shutdown.

Now the slot will be disconnected on destruction.

Change-Id: Ie511bf5400c41eee3245879ea355293e7bb5c7cd
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/clangcodemodel/clangbackendipcintegration.h

@@ -143,6 +143,7 @@ private:
 
     void onBackendRestarted();
     void onEditorAboutToClose(Core::IEditor *editor);
+    void onCoreAboutToClose();
 
     IpcReceiver m_ipcReceiver;
     ClangBackEnd::ConnectionClient m_connection;