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/cppeditor/cppeditordocument.cpp

@@ -257,6 +257,8 @@ void CppEditorDocument::scheduleProcessDocument()
 
 void CppEditorDocument::processDocument()
 {
+    processor()->invalidateDiagnostics();
+
     if (processor()->isParserRunning() || m_processorRevision != contentsRevision()) {
         m_processorTimer.start();
         processor()->editorDocumentTimerRestarted();