[SPARK-6145][SQL] fix ORDER BY on nested fields

Based on #4904 with style errors fixed.

`LogicalPlan#resolve` will not only produce `Attribute`, but also "`GetField` chain".
So in `ResolveSortReferences`, after resolve the ordering expressions, we should not just collect the `Attribute` results, but also `Attribute` at the bottom of "`GetField` chain".

Author: Wenchen Fan <cloud0fan@outlook.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #4918 from marmbrus/pr/4904 and squashes the following commits:

997f84e [Michael Armbrust] fix style
3eedbfc [Wenchen Fan] fix 6145