[SPARK-12727][SQL] support SQL generation for aggregate with multi-distinct
## What changes were proposed in this pull request? This PR add SQL generation support for aggregate with multi-distinct, by simply moving the `DistinctAggregationRewriter` rule to optimizer. More discussions are needed as this breaks an import contract: analyzed plan should be able to run without optimization. However, the `ComputeCurrentTime` rule has kind of broken it already, and I think maybe we should add a new phase for this kind of rules, because strictly speaking they don't belong to analysis and is coupled with the physical plan implementation. ## How was this patch tested? existing tests Author: Wenchen Fan <wenchen@databricks.com> Closes #11579 from cloud-fan/distinct.
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