Clang: Add output argument highlighting

This adds a mix-in for writable functions arguments.

Change-Id: I758f7fef77d992ea25395db550571ccb081fd5fd
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clanghighlightingmarksreporter.cpp

@@ -60,6 +60,8 @@ TextEditor::TextStyle toTextStyle(ClangBackEnd::HighlightingType type)
             return TextEditor::C_PREPROCESSOR;
         case HighlightingType::Declaration:
             return TextEditor::C_DECLARATION;
+        case HighlightingType::OutputArgument:
+            return TextEditor::C_OUTPUT_ARGUMENT;
         default:
             return TextEditor::C_TEXT; // never called
     }