[SPARK-18869][SQL] Add TreeNode.p that returns BaseType

## What changes were proposed in this pull request?
After the bug fix in SPARK-18854, TreeNode.apply now returns TreeNode[_] rather than a more specific type. It would be easier for interactive debugging to introduce a function that returns the BaseType.

## How was this patch tested?
N/A - this is a developer only feature used for interactive debugging. As long as it compiles, it should be good to go. I tested this in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16288 from rxin/SPARK-18869.