[SPARK-22848][SQL] Eliminate mutable state from Stack

## What changes were proposed in this pull request?

This PR eliminates mutable states from the generated code for `Stack`.

## How was this patch tested?

Existing test suites

Author: Kazuaki Ishizaki <[email protected]>

Closes #20035 from kiszk/SPARK-22848.

Author: kiszk

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/generators.scala

@@ -214,11 +214,11 @@ case class Stack(children: Seq[Expression]) extends Generator {
 
     // Create the collection.
     val wrapperClass = classOf[mutable.WrappedArray[_]].getName
-    ctx.addMutableState(
-      s"$wrapperClass<InternalRow>",
-      ev.value,
-      v => s"$v = $wrapperClass$$.MODULE$$.make($rowData);", useFreshName = false)
-    ev.copy(code = code, isNull = "false")
+    ev.copy(code =
+      s"""
+         |$code
+         |$wrapperClass<InternalRow> ${ev.value} = $wrapperClass$$.MODULE$$.make($rowData);
+       """.stripMargin, isNull = "false")
   }
 }