[SPARK-14562] [SQL] improve constraints propagation in Union

## What changes were proposed in this pull request?

Currently, Union only takes intersect of the constraints from it's children, all others are dropped, we should try to merge them together.

This PR try to merge the constraints that have the same reference but came from different children, for example: `a > 10` and `a < 100` could be merged as `a > 10 || a < 100`.

## How was this patch tested?

Added more cases in existing test.

Author: Davies Liu <davies@databricks.com>

Closes #12328 from davies/union_const.