[SPARK-9241][SQL] Supporting multiple DISTINCT columns - follow-up (3)

This PR is a 2nd follow-up for [SPARK-9241](https://issues.apache.org/jira/browse/SPARK-9241). It contains the following improvements:
* Fix for a potential bug in distinct child expression and attribute alignment.
* Improved handling of duplicate distinct child expressions.
* Added test for distinct UDAF with multiple children.

cc yhuai

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #9566 from hvanhovell/SPARK-9241-followup-2.