## What changes were proposed in this pull request?
When the pivot column is of a complex type, the eval() result will be an UnsafeRow, while the keys of the HashMap for column value matching is a GenericInternalRow. As a result, there will be no match and the result will always be empty.
So for a pivot column of complex-types, we should:
1) If the complex-type is not comparable (orderable), throw an Exception. It cannot be a pivot column.
2) Otherwise, if it goes through the `PivotFirst` code path, `PivotFirst` should use a TreeMap instead of HashMap for such columns.
This PR has also reverted the walk-around in Analyzer that had been introduced to avoid this `PivotFirst` issue.
## How was this patch tested?
Added UT.
Author: maryannxue <maryannxue@apache.org>
Closes#21926 from maryannxue/pivot_followup.
b4fd75fb9b