[SPARK-11646] WholeTextFileRDD should return Text rather than String

[rxin]

If it returns Text, we can reuse this in Spark SQL to provide a WholeTextFile data source and directly convert the Text into UTF8String without extra string decoding and encoding.

[SparkQA]

Test build #45610 has finished for PR 9622 at commit 4d412dd.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaLBFGSExample\n * class LDA @Since(\"1.6.0\") (\n * case class Metadata(\n * require(className == expectedClassName, s\"Error loading metadata: Expected class name\" +\n * class SlidingRDDPartition[T](val idx: Int, val prev: Partition, val tail: Seq[T], val offset: Int)\n * class SlidingRDD[T: ClassTag](@transient val parent: RDD[T], val windowSize: Int, val step: Int)\n

[JoshRosen]

Jenkins, retest this please.

[srowen]

LGTM since it's internal

[SparkQA]

Test build #45617 has finished for PR 9622 at commit 4d412dd.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[davies]

Merging this into master and 1.6.

[rxin]

(racing. I just merge this too)