[SPARK-14360][SQL] QueryExecution.debug.codegen() to dump codegen

## What changes were proposed in this pull request?
We recently added the ability to dump the generated code for a given query. However, the method is only available through an implicit after an import. It'd slightly simplify things if it can be called directly in queryExecution.

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

Author: Reynold Xin <rxin@databricks.com>

Closes #12144 from rxin/SPARK-14360.

sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala

@@ -103,4 +103,20 @@ class QueryExecution(val sqlContext: SQLContext, val logical: LogicalPlan) {
        |${stringOrError(executedPlan)}
     """.stripMargin.trim
   }
+
+  /** A special namespace for commands that can be used to debug query execution. */
+  // scalastyle:off
+  object debug {
+  // scalastyle:on
+
+    /**
+     * Prints to stdout all the generated code found in this plan (i.e. the output of each
+     * WholeStageCodegen subtree).
+     */
+    def codegen(): Unit = {
+      // scalastyle:off println
+      println(org.apache.spark.sql.execution.debug.codegenString(executedPlan))
+      // scalastyle:on println
+    }
+  }
 }