CompilationDB: Reparse only on actual project file change

Fixes: QTCREATORBUG-22574
Change-Id: I39fe58f96c1ff9118405be225f39e5348304222e
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseproject.h

@@ -70,6 +70,7 @@ private:
     std::unique_ptr<CppTools::CppProjectUpdater> m_cppCodeModelUpdater;
     std::unique_ptr<ProjectExplorer::Kit> m_kit;
     MimeBinaryCache m_mimeBinaryCache;
+    QByteArray m_projectFileHash;
     QTimer * const m_parseDelay;
     CompilationDbParser *m_parser = nullptr;
 };