Expand allowed order by expressions

with a non-simple query we won't allow derived expressions in the order by such as:

select max(x), y from t group by y order by case when y is null then 0 else y end

We treat this ordering as unrelated - however it is expressed in terms of expressions from the select clause. Mondrian will generate queries of this form, which requires a workaround of modifying the dialect to use ansi null ordering.