CppTools: Tests: Fix crash in test_modelmanager_gc_if_last_cppeditor_closed

The project that was referenced at cppeditor.cpp:711 was already
deleted. CppModelManager::m_fileToProjectParts contained outdated
information. Now it is updated if a project is closed.

Exposed by commit d411c07.

Change-Id: I6fee93a02c72a045dd44a05723ab39a1eeec616e
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cppmodelmanager.h

@@ -170,6 +170,8 @@ private slots:
 
 private:
     void delayedGC();
+    void recalculateFileToProjectParts();
+
     void replaceSnapshot(const CPlusPlus::Snapshot &newSnapshot);
     void removeFilesFromSnapshot(const QSet<QString> &removedFiles);
     void removeProjectInfoFilesAndIncludesFromSnapshot(const ProjectInfo &projectInfo);