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/texteditor/texteditorconstants.h

@@ -100,6 +100,7 @@ enum TextStyle : quint8 {
     C_ERROR_CONTEXT,
 
     C_DECLARATION,
+    C_OUTPUT_ARGUMENT,
 
     C_LAST_STYLE_SENTINEL
 };