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/clangeditordocumentprocessor.h

@@ -77,6 +77,7 @@ public:
     TextEditor::QuickFixOperations
     extraRefactoringOperations(const TextEditor::AssistInterface &assistInterface) override;
 
+    void invalidateDiagnostics() override;
     bool hasDiagnosticsAt(uint line, uint column) const override;
     void addDiagnosticToolTipToLayout(uint line, uint column, QLayout *target) const override;