[SPARK-18714][SQL] Add a simple time function to SparkSession

## What changes were proposed in this pull request?
Many Spark developers often want to test the runtime of some function in interactive debugging and testing. This patch adds a simple time function to SparkSession:

```
scala> spark.time { spark.range(1000).count() }
Time taken: 77 ms
res1: Long = 1000
```

## How was this patch tested?
I tested this interactively in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16140 from rxin/SPARK-18714.

(cherry picked from commit cb1f10b4)
Signed-off-by: Herman van Hovell <hvanhovell@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/SparkSession.scala

@@ -618,6 +618,22 @@ class SparkSession private(
   @InterfaceStability.Evolving
   def readStream: DataStreamReader = new DataStreamReader(self)
 
+  /**
+   * Executes some code block and prints to stdout the time taken to execute the block. This is
+   * available in Scala only and is used primarily for interactive testing and debugging.
+   *
+   * @since 2.1.0
+   */
+  @InterfaceStability.Stable
+  def time[T](f: => T): T = {
+    val start = System.nanoTime()
+    val ret = f
+    val end = System.nanoTime()
+    // scalastyle:off println
+    println(s"Time taken: ${(end - start) / 1000 / 1000} ms")
+    // scalastyle:on println
+    ret
+  }
 
   // scalastyle:off
   // Disable style checker so "implicits" object can start with lowercase i