[SPARK-16648][SQL] Make ignoreNullsExpr a child expression of First and Last

## What changes were proposed in this pull request?

Default `TreeNode.withNewChildren` implementation doesn't work for `Last` and when both constructor arguments are the same, e.g.:

```sql
LAST_VALUE(FALSE) -- The 2nd argument defaults to FALSE
LAST_VALUE(FALSE, FALSE)
LAST_VALUE(TRUE, TRUE)
```

This is because although `Last` is a unary expression, both of its constructor arguments, `child` and `ignoreNullsExpr`, are `Expression`s. When they have the same value, `TreeNode.withNewChildren` treats both of them as child nodes by mistake. `First` is also affected by this issue in exactly the same way.

This PR fixes this issue by making `ignoreNullsExpr` a child expression of `First` and `Last`.

## How was this patch tested?

New test case added in `WindowQuerySuite`.

Author: Cheng Lian <lian@databricks.com>

Closes #14295 from liancheng/spark-16648-last-value.

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

@@ -43,7 +43,7 @@ case class First(child: Expression, ignoreNullsExpr: Expression) extends Declara
       throw new AnalysisException("The second argument of First should be a boolean literal.")
   }
 
-  override def children: Seq[Expression] = child :: Nil
+  override def children: Seq[Expression] = child :: ignoreNullsExpr :: Nil
 
   override def nullable: Boolean = true
 
@@ -54,7 +54,7 @@ case class First(child: Expression, ignoreNullsExpr: Expression) extends Declara
   override def dataType: DataType = child.dataType
 
   // Expected input data type.
-  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType)
+  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType, BooleanType)
 
   private lazy val first = AttributeReference("first", child.dataType)()
 

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

@@ -40,7 +40,7 @@ case class Last(child: Expression, ignoreNullsExpr: Expression) extends Declarat
       throw new AnalysisException("The second argument of First should be a boolean literal.")
   }
 
-  override def children: Seq[Expression] = child :: Nil
+  override def children: Seq[Expression] = child :: ignoreNullsExpr :: Nil
 
   override def nullable: Boolean = true
 
@@ -51,7 +51,7 @@ case class Last(child: Expression, ignoreNullsExpr: Expression) extends Declarat
   override def dataType: DataType = child.dataType
 
   // Expected input data type.
-  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType)
+  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType, BooleanType)
 
   private lazy val last = AttributeReference("last", child.dataType)()
 

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowQuerySuite.scala

@@ -247,4 +247,16 @@ class WindowQuerySuite extends QueryTest with SQLTestUtils with TestHiveSingleto
         |from part
         """.stripMargin))
   }
+
+  test("SPARK-16646: LAST_VALUE(FALSE) OVER ()") {
+    checkAnswer(sql("SELECT LAST_VALUE(FALSE) OVER ()"), Row(false))
+    checkAnswer(sql("SELECT LAST_VALUE(FALSE, FALSE) OVER ()"), Row(false))
+    checkAnswer(sql("SELECT LAST_VALUE(TRUE, TRUE) OVER ()"), Row(true))
+  }
+
+  test("SPARK-16646: FIRST_VALUE(FALSE) OVER ()") {
+    checkAnswer(sql("SELECT FIRST_VALUE(FALSE) OVER ()"), Row(false))
+    checkAnswer(sql("SELECT FIRST_VALUE(FALSE, FALSE) OVER ()"), Row(false))
+    checkAnswer(sql("SELECT FIRST_VALUE(TRUE, TRUE) OVER ()"), Row(true))
+  }
 }