[SPARK-4487][SQL] Fix attribute reference resolution error when using ORDER BY.

When we use ORDER BY clause, at first, attributes referenced by projection are resolved (1).
And then, attributes referenced at ORDER BY clause are resolved (2).
 But when resolving attributes referenced at ORDER BY clause, the resolution result generated in (1) is discarded so for example, following query fails.

    SELECT c1 + c2 FROM mytable ORDER BY c1;

The query above fails because when resolving the attribute reference 'c1', the resolution result of 'c2' is discarded.
projectList.map(_.toAttribute) ?