[SPARK-13310] [SQL] Resolve Missing Sorting Columns in Generate

```scala
// case 1: missing sort columns are resolvable if join is true
sql("SELECT explode(a) AS val, b FROM data WHERE b < 2 order by val, c")
// case 2: missing sort columns are not resolvable if join is false. Thus, issue an error message in this case
sql("SELECT explode(a) AS val FROM data order by val, c")
```

When sort columns are not in `Generate`, we can resolve them when `join` is equal to `true`. Still trying to add more test cases for the other `UnaryNode` types.

Could you review the changes? davies cloud-fan Thanks!

Author: gatorsmile <gatorsmile@gmail.com>

Closes #11198 from gatorsmile/missingInSort.