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When the editor is closed, the CppEditorSupport waits for the
highlighting and semantic info futures to finish. These futures might
access the CppEditorSupport through the CppModelManager, causing a
deadlock on the m_editorSupportMutex.

Change-Id: Ifeb3864ed3bc2666d83607ef50d7bfee8f3d118f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>