[SPARK-13721][SQL] Make GeneratorOuter unresolved.

## What changes were proposed in this pull request?

This is a small change to make GeneratorOuter always unresolved. It is mostly no-op change but makes it more clear since GeneratorOuter shouldn't survive analysis phase.
This requires also handling in ResolveAliases rule.

## How was this patch tested?
Existing generator tests.

Author: Bogdan Raducanu <bogdan@databricks.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #17026 from bogdanrdc/PR16958.