CppEditor: First un-register, then register in CppModelManager

...on a file path change.

Change-Id: Id4f67c9f133e53ddb3fea6c0ec3322f87914469b
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/cppeditor/cppeditordocument.cpp

@@ -207,6 +207,7 @@ void CppEditorDocument::onFilePathChanged(const QString &oldPath, const QString
         connect(this, SIGNAL(contentsChanged()), this, SLOT(scheduleProcessDocument()));
 
         // Un-Register/Register in ModelManager
+        m_editorDocumentHandle.reset();
         m_editorDocumentHandle.reset(new CppEditorDocumentHandle(this));
 
         resetProcessor();