ClangCodeModel: Clear existing libclang diagnostics

... when clangd takes over a document.
This fixes the problem that temporary diagnostics persisted after opening
a session.

Change-Id: Ic781f83747cabb4d15c2c1f8181e36f4343e0394
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdiagnosticmanager.h

@@ -65,11 +65,11 @@ public:
 
     static void clearTaskHubIssues();
     void generateTaskHubIssues();
+    void cleanMarks();
 
     static void addTask(const ClangBackEnd::DiagnosticContainer &diagnostic, bool isChild = false);
 
 private:
-    void cleanMarks();
     QString filePath() const;
     void filterDiagnostics(const QVector<ClangBackEnd::DiagnosticContainer> &diagnostics);
     void generateEditorSelections();