Clang: Gray out diagnostics on document change

When reparses take a while, this is helpful.

Change-Id: Ie2003a3d65b30d944d20fa19dd4161412182851c
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangtextmark.h

@@ -43,8 +43,8 @@ public:
                   const ClangBackEnd::DiagnosticContainer &diagnostic,
                   const RemovedFromEditorHandler &removedHandler);
 
+    void updateIcon(bool valid = true);
 private:
-    void setIcon(ClangBackEnd::DiagnosticSeverity severity);
     bool addToolTipContent(QLayout *target) const override;
     void removedFromEditor() override;