[SPARK-3204][SQL] MaxOf would be foldable if both left and right are foldable.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2116 from ueshin/issues/SPARK-3204 and squashes the following commits:

7d9b107 [Takuya UESHIN] Make MaxOf foldable if both left and right are foldable.

(cherry picked from commit d299e2bf2f6733a6267b7ce85e2b288608b17db3)
Signed-off-by: Michael Armbrust <michael@databricks.com>