[SPARK-18869][SQL] Add TreeNode.p that returns BaseType

## What changes were proposed in this pull request?
After the bug fix in SPARK-18854, TreeNode.apply now returns TreeNode[_] rather than a more specific type. It would be easier for interactive debugging to introduce a function that returns the BaseType.

## How was this patch tested?
N/A - this is a developer only feature used for interactive debugging. As long as it compiles, it should be good to go. I tested this in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16288 from rxin/SPARK-18869.

(cherry picked from commit 5d510c693aca8c3fd3364b4453160bc8585ffc8e)
Signed-off-by: Reynold Xin <rxin@databricks.com>

Author: rxin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala

@@ -24,15 +24,6 @@ import org.apache.spark.sql.types.{DataType, StructType}
 abstract class QueryPlan[PlanType <: QueryPlan[PlanType]] extends TreeNode[PlanType] {
   self: PlanType =>
 
-  /**
-   * Override [[TreeNode.apply]] to so we can return a more narrow type.
-   *
-   * Note that this cannot return BaseType because logical plan's plan node might return
-   * physical plan for innerChildren, e.g. in-memory relation logical plan node has a reference
-   * to the physical plan node it is referencing.
-   */
-  override def apply(number: Int): QueryPlan[_] = super.apply(number).asInstanceOf[QueryPlan[_]]
-
   def output: Seq[Attribute]
 
   /**

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -498,7 +498,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
     treeString.split("\n").zipWithIndex.map { case (line, i) => f"$i%02d $line" }.mkString("\n")
 
   /**
-   * Returns the tree node at the specified number.
+   * Returns the tree node at the specified number, used primarily for interactive debugging.
    * Numbers for each node can be found in the [[numberedTreeString]].
    *
    * Note that this cannot return BaseType because logical plan's plan node might return
@@ -507,6 +507,14 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
    */
   def apply(number: Int): TreeNode[_] = getNodeNumbered(new MutableInt(number)).orNull
 
+  /**
+   * Returns the tree node at the specified number, used primarily for interactive debugging.
+   * Numbers for each node can be found in the [[numberedTreeString]].
+   *
+   * This is a variant of [[apply]] that returns the node as BaseType (if the type matches).
+   */
+  def p(number: Int): BaseType = apply(number).asInstanceOf[BaseType]
+
   private def getNodeNumbered(number: MutableInt): Option[TreeNode[_]] = {
     if (number.i < 0) {
       None