[SPARK-19338][SQL] Add UDF names in explain

## What changes were proposed in this pull request?
This pr added a variable for a UDF name in `ScalaUDF`.
Then, if the variable filled, `DataFrame#explain` prints the name.

## How was this patch tested?
Added a test in `UDFSuite`.

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #16707 from maropu/SPARK-19338.

Author: maropu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala

@@ -2071,7 +2071,7 @@ class Analyzer(
 
       case p => p transformExpressionsUp {
 
-        case udf @ ScalaUDF(func, _, inputs, _) =>
+        case udf @ ScalaUDF(func, _, inputs, _, _) =>
           val parameterTypes = ScalaReflection.getParameterTypes(func)
           assert(parameterTypes.length == inputs.length)
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ScalaUDF.scala

@@ -35,17 +35,20 @@ import org.apache.spark.sql.types.DataType
  *                    not want to perform coercion, simply use "Nil". Note that it would've been
  *                    better to use Option of Seq[DataType] so we can use "None" as the case for no
  *                    type coercion. However, that would require more refactoring of the codebase.
+ * @param udfName   The user-specified name of this UDF.
  */
 case class ScalaUDF(
     function: AnyRef,
     dataType: DataType,
     children: Seq[Expression],
-    inputTypes: Seq[DataType] = Nil)
+    inputTypes: Seq[DataType] = Nil,
+    udfName: Option[String] = None)
   extends Expression with ImplicitCastInputTypes with NonSQLExpression {
 
   override def nullable: Boolean = true
 
-  override def toString: String = s"UDF(${children.mkString(", ")})"
+  override def toString: String =
+    s"${udfName.map(name => s"UDF:$name").getOrElse("UDF")}(${children.mkString(", ")})"
 
   // scalastyle:off line.size.limit