Try harder to get context help in presence of diagnostics

- even if there are diagnostics still try to retrieve symbol info
  and help
- fall back to text based keyword extraction in case code model
  info fails
- if both a code model tool tip (e.g. function signature or type)
  and help are available, show both

Task-number: QTCREATORBUG-15959
Change-Id: Id85a223c24849ead1b25d63776d64a7da1cc73ef
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cpphoverhandler.h

@@ -40,7 +40,6 @@ private:
     void identifyMatch(TextEditor::TextEditorWidget *editorWidget,
                        int pos,
                        ReportPriority report) override;
-    void decorateToolTip() override;
 };
 
 } // namespace CppTools