[SPARK-18969][SQL] Support grouping by nondeterministic expressions

## What changes were proposed in this pull request?

Currently nondeterministic expressions are allowed in `Aggregate`(see the [comment](https://github.com/apache/spark/blob/v2.0.2/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CheckAnalysis.scala#L249-L251)), but the `PullOutNondeterministic` analyzer rule failed to handle `Aggregate`, this PR fixes it.

close https://github.com/apache/spark/pull/16379

There is still one remaining issue: `SELECT a + rand() FROM t GROUP BY a + rand()` is not allowed, because the 2 `rand()` are different(we generate random seed as the default seed for `rand()`). https://issues.apache.org/jira/browse/SPARK-19035

 is tracking this issue.

## How was this patch tested?

a new test suite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16404 from cloud-fan/groupby.

(cherry picked from commit 871d2666)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>