[SPARK-13236] SQL Generation for Set Operations

This PR is to implement SQL generation for the following three set operations:
- Union Distinct
- Intersect
- Except

liancheng Thanks!

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

Closes #11195 from gatorsmile/setOpSQLGen.