Add flag for semantic highlighter to set diagnostics.

If the semantic highlighter does not take care of setting showing the wiggly
lines for diagnostics in the editor, the model manager will do.

Change-Id: Ie69fb798dd53d60ddca1668b8f586266a0daca4b
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/plugins/cpptools/cpphighlightingsupport.h

@@ -72,6 +72,8 @@ public:
     virtual ~CppHighlightingSupportFactory() = 0;
 
     virtual CppHighlightingSupport *highlightingSupport(TextEditor::ITextEditor *editor) = 0;
+
+    virtual bool hightlighterHandlesDiagnostics() const = 0;
 };
 
 } // namespace CppTools