[SPARK-18216][SQL] Make Column.expr public

## What changes were proposed in this pull request?
Column.expr is private[sql], but it's an actually really useful field to have for debugging. We should open it up, similar to how we use QueryExecution.

## How was this patch tested?
N/A - this is a simple visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #15724 from rxin/SPARK-18216.