[SPARK-11596][SQL] In TreeNode's argString, if a TreeNode is not a child of the current TreeNode, we should only return the simpleString.

In TreeNode's argString, if a TreeNode is not a child of the current TreeNode, we will only return the simpleString.

I tested the [following case provided by Cristian](https://issues.apache.org/jira/browse/SPARK-11596?focusedCommentId=15019241&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15019241).
```
val c = (1 to 20).foldLeft[Option[DataFrame]] (None) { (curr, idx) =>
    println(s"PROCESSING >>>>>>>>>>> $idx")
    val df = sqlContext.sparkContext.parallelize((0 to 10).zipWithIndex).toDF("A", "B")
    val union = curr.map(_.unionAll(df)).getOrElse(df)
    union.cache()
    Some(union)
  }

c.get.explain(true)
```

Without the change, `c.get.explain(true)` took 100s. With the change, `c.get.explain(true)` took 26ms.

https://issues.apache.org/jira/browse/SPARK-11596

Author: Yin Huai <yhuai@databricks.com>

Closes #10079 from yhuai/SPARK-11596.

Author: yhuai

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

@@ -380,7 +380,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   /** Returns a string representing the arguments to this node, minus any children */
   def argString: String = productIterator.flatMap {
     case tn: TreeNode[_] if containsChild(tn) => Nil
-    case tn: TreeNode[_] if tn.toString contains "\n" => s"(${tn.simpleString})" :: Nil
+    case tn: TreeNode[_] => s"(${tn.simpleString})" :: Nil
     case seq: Seq[BaseType] if seq.toSet.subsetOf(children.toSet) => Nil
     case seq: Seq[_] => seq.mkString("[", ",", "]") :: Nil
     case set: Set[_] => set.mkString("{", ",", "}") :: Nil