[SPARK-17618] Guard against invalid comparisons between UnsafeRow and other formats

This patch ports changes from #15185 to Spark 2.x. In that patch, a  correctness bug in Spark 1.6.x which was caused by an invalid `equals()` comparison between an `UnsafeRow` and another row of a different format. Spark 2.x is not affected by that specific correctness bug but it can still reap the error-prevention benefits of that patch's changes, which modify  ``UnsafeRow.equals()` to throw an IllegalArgumentException if it is called with an object that is not an `UnsafeRow`.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #15265 from JoshRosen/SPARK-17618-master.