[SPARK-42346][SQL] Rewrite distinct aggregates after subquery merge

### What changes were proposed in this pull request?

Unfortunately #32298 introduced a regression from Spark 3.2 to 3.3 as after that change a merged subquery can contain multiple distict type aggregates. Those aggregates need to be rewritten by the `RewriteDistinctAggregates` rule to get the correct results. This PR fixed that.

### Why are the changes needed?
The following query:
```
SELECT
  (SELECT count(distinct c1) FROM t1),
  (SELECT count(distinct c2) FROM t1)
```
currently fails with:
```
java.lang.IllegalStateException: You hit a query analyzer bug. Please report your query to Spark user mailing list.
	at org.apache.spark.sql.execution.SparkStrategies$Aggregation$.apply(SparkStrategies.scala:538)
```
but works again after this PR.

### Does this PR introduce _any_ user-facing change?
Yes, the above query works again.

### How was this patch tested?
Added new UT.

Closes #39887 from peter-toth/SPARK-42346-rewrite-distinct-aggregates-after-subquery-merge.

Authored-by: Peter Toth <[email protected]>
Signed-off-by: Yuming Wang <[email protected]>

Author: peter-toth

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala

@@ -58,7 +58,8 @@ class SparkOptimizer(
     Batch("InjectRuntimeFilter", FixedPoint(1),
       InjectRuntimeFilter) :+
     Batch("MergeScalarSubqueries", Once,
-      MergeScalarSubqueries) :+
+      MergeScalarSubqueries,
+      RewriteDistinctAggregates) :+
     Batch("Pushdown Filters from PartitionPruning", fixedPoint,
       PushDownPredicates) :+
     Batch("Cleanup filters that cannot be pushed down", Once,

sql/core/src/test/scala/org/apache/spark/sql/SubquerySuite.scala

@@ -2598,4 +2598,29 @@ class SubquerySuite extends QueryTest
         Row("aa"))
     }
   }
+
+  test("SPARK-42346: Rewrite distinct aggregates after merging subqueries") {
+    withTempView("t1") {
+      Seq((1, 2), (3, 4)).toDF("c1", "c2").createOrReplaceTempView("t1")
+
+      checkAnswer(sql(
+        """
+          |SELECT
+          |  (SELECT count(distinct c1) FROM t1),
+          |  (SELECT count(distinct c2) FROM t1)
+          |""".stripMargin),
+        Row(2, 2))
+
+      // In this case we don't merge the subqueries as `RewriteDistinctAggregates` kicks off for the
+      // 2 subqueries first but `MergeScalarSubqueries` is not prepared for the `Expand` nodes that
+      // are inserted by the rewrite.
+      checkAnswer(sql(
+        """
+          |SELECT
+          |  (SELECT count(distinct c1) + sum(distinct c2) FROM t1),
+          |  (SELECT count(distinct c2) + sum(distinct c1) FROM t1)
+          |""".stripMargin),
+        Row(8, 6))
+    }
+  }
 }