C++: do not run GC when exiting.

This operation is quite costly if a lot of files are involved, and in
case of exiting Creator, it's also useless.

Change-Id: I97d178d47a3a2f6b214f7ebc45c871edd26b8286
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/cpptools/cppmodelmanager.h

@@ -171,6 +171,7 @@ private Q_SLOTS:
     void onExtraDiagnosticsUpdated(const QString &fileName);
     void onAboutToRemoveProject(ProjectExplorer::Project *project);
     void onAboutToUnloadSession();
+    void onCoreAboutToClose();
     void onProjectAdded(ProjectExplorer::Project *project);
     void postEditorUpdate();
     void updateEditorSelections();
@@ -195,6 +196,7 @@ private:
 
 private:
     CPlusPlus::Snapshot m_snapshot;
+    bool m_enableGC;
 
     // cache
     bool m_dirty;