ClangTools: Fix showing diagnostics outside the project root dir

Accept diagnostics from files that are known to the project
(Project::files()) instead of checking whether they are below the
project root directory (Project::projectDirectory()).

Fixes: QTCREATORBUG-22213
Change-Id: I2f96374d6de3f53b1bd42be875dfe44b25a55fb1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangtools/clangtoolruncontrol.h

@@ -104,6 +104,7 @@ private:
 
     QFutureInterface<void> m_progress;
     AnalyzeUnits m_unitsToProcess;
+    QSet<Utils::FilePath> m_projectFiles;
     QSet<ClangToolRunner *> m_runners;
     int m_initialFilesToProcessSize = 0;
     int m_filesAnalyzed = 0;