[SPARK-16855][SQL] move Greatest and Least from conditionalExpressions.scala to arithmetic.scala

[cloud-fan]

What changes were proposed in this pull request?

Greatest and Least are not conditional expressions, but arithmetic expressions.

How was this patch tested?

N/A

[SparkQA]

Test build #63119 has finished for PR 14460 at commit a2e03ca.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class Least(children: Seq[Expression]) extends Expression
  • case class Greatest(children: Seq[Expression]) extends Expression

[rxin]

Merging in master/2.0.

[rxin]

Actually I've only merged this in master.