Clang: Show info bar for parse errors in header files

...because those errors can lead to a substantial performance/functional
regression.

The actual diagnostics (possibly with children) are shown as details in
the info bar.

The info bar can be hidden with the "Do Not Show Again" button.
Re-enabling the info bar is possible with the new editor tool bar
button.

Change-Id: I03394ff8e3c84127946b0b791930b28a385f5a46
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/baseeditordocumentprocessor.h

@@ -37,6 +37,8 @@
 
 #include <QTextEdit>
 
+#include <functional>
+
 namespace TextEditor {
 class TextDocument;
 class QuickFixOperations;
@@ -69,10 +71,15 @@ public:
 
     virtual void editorDocumentTimerRestarted();
 
+public:
+    using HeaderErrorDiagnosticWidgetCreator = std::function<QWidget*()>;
+
 signals:
+
     // Signal interface to implement
     void codeWarningsUpdated(unsigned revision,
                              const QList<QTextEdit::ExtraSelection> selections,
+                             const HeaderErrorDiagnosticWidgetCreator &creator,
                              const TextEditor::RefactorMarkers &refactorMarkers);
 
     void ifdefedOutBlocksUpdated(unsigned revision,