[SPARK-16724] Expose DefinedByConstructorParams

We don't generally make things in catalyst/execution private.  Instead they are just undocumented due to their lack of stability guarantees.

Author: Michael Armbrust <michael@databricks.com>

Closes #14356 from marmbrus/patch-1.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -30,7 +30,7 @@ import org.apache.spark.unsafe.types.{CalendarInterval, UTF8String}
  * for classes whose fields are entirely defined by constructor params but should not be
  * case classes.
  */
-private[sql] trait DefinedByConstructorParams
+trait DefinedByConstructorParams
 
 
 /**