[SPARK-9784] [SQL] Exchange.isUnsafe should check whether codegen and unsafe are enabled

Exchange.isUnsafe should check whether codegen and unsafe are enabled.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #8073 from JoshRosen/SPARK-9784 and squashes the following commits:

7a1019f [Josh Rosen] [SPARK-9784] Exchange.isUnsafe should check whether codegen and unsafe are enabled

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

@@ -46,7 +46,7 @@ case class Exchange(newPartitioning: Partitioning, child: SparkPlan) extends Una
    * Returns true iff we can support the data type, and we are not doing range partitioning.
    */
   private lazy val tungstenMode: Boolean = {
-    GenerateUnsafeProjection.canSupport(child.schema) &&
+    unsafeEnabled && codegenEnabled && GenerateUnsafeProjection.canSupport(child.schema) &&
       !newPartitioning.isInstanceOf[RangePartitioning]
   }