[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.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala

@@ -89,6 +89,8 @@ case class Remainder(left: Expression, right: Expression) extends BinaryArithmet
 case class MaxOf(left: Expression, right: Expression) extends Expression {
   type EvaluatedType = Any
 
+  override def foldable = left.foldable && right.foldable
+
   override def nullable = left.nullable && right.nullable
 
   override def children = left :: right :: Nil