[SPARK-26084][SQL] Fixes AggregateExpression.references exception

Implements the solution proposed in [SPARK-26084](https://issues.apache.org/jira/browse/SPARK-26084),
a minor refactoring that removes the unnecessary dependence on `AttributeSet.toSeq`,
which requires expression IDs and, therefore, can only execute successfully for resolved expressions.

The refactored implementation is both simpler and faster, eliminating the conversion of a `Set` to a
`Seq` and back to `Set`.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/interfaces.scala

@@ -128,12 +128,10 @@ case class AggregateExpression(
   override def nullable: Boolean = aggregateFunction.nullable
 
   override def references: AttributeSet = {
-    val childReferences = mode match {
-      case Partial | Complete => aggregateFunction.references.toSeq
-      case PartialMerge | Final => aggregateFunction.aggBufferAttributes
+    mode match {
+      case Partial | Complete => aggregateFunction.references
+      case PartialMerge | Final => AttributeSet(aggregateFunction.aggBufferAttributes)
     }
-
-    AttributeSet(childReferences)
   }
 
   override def toString: String = {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/aggregate/AggregateExpressionSuite.scala

@@ -0,0 +1,34 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.expressions.aggregate
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute
+import org.apache.spark.sql.catalyst.expressions.{Add, AttributeSet}
+
+class AggregateExpressionSuite extends SparkFunSuite {
+
+  test("test references from unresolved aggregate functions") {
+    val x = UnresolvedAttribute("x")
+    val y = UnresolvedAttribute("y")
+    val actual = AggregateExpression(Sum(Add(x, y)), mode = Complete, isDistinct = false).references
+    val expected = AttributeSet(x :: y :: Nil)
+    assert(expected == actual, s"Expected: $expected. Actual: $actual")
+  }
+
+}