[SPARK-2911] apply parent[T](j) to clarify UnionRDD code

References to dependencies(j) for actually obtaining RDD parents are less common than I originally estimated.   It does clarify UnionRDD (also will clarify some of my other PRs)

Use of firstParent[T] is ubiquitous, but not as sure that benefits from being replaced with parent(0)[T].

Author: Erik Erlandson <eerlands@redhat.com>

Closes #1858 from erikerlandson/spark-2911-pr2 and squashes the following commits:

7ffea74 [Erik Erlandson] [SPARK-2911] apply parent[T](j) to clarify UnionRDD code

core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala

@@ -83,8 +83,7 @@ class UnionRDD[T: ClassTag](
 
   override def compute(s: Partition, context: TaskContext): Iterator[T] = {
     val part = s.asInstanceOf[UnionPartition[T]]
-    val parentRdd = dependencies(part.parentRddIndex).rdd.asInstanceOf[RDD[T]]
-    parentRdd.iterator(part.parentPartition, context)
+    parent[T](part.parentRddIndex).iterator(part.parentPartition, context)
   }
 
   override def getPreferredLocations(s: Partition): Seq[String] =