[SPARK-8572] [SQL] Type coercion for ScalaUDFs

Implemented type coercion for udf arguments in Scala. The changes include-
* Add `with ExpectsInputTypes ` to `ScalaUDF` class.
* Pass down argument types info from `UDFRegistration` and `functions`.

With this patch, the example query in [SPARK-8572](https://issues.apache.org/jira/browse/SPARK-8572) no longer throws a type cast error at runtime.

Also added a unit test to `UDFSuite` in which a decimal type is passed to a udf that expects an int.

Author: Cheolsoo Park <[email protected]>

Closes #7203 from piaozhexiu/SPARK-8572 and squashes the following commits:

2d0ed15 [Cheolsoo Park] Incorporate comments
dce1efd [Cheolsoo Park] Fix unit tests and update the codegen script
066deed [Cheolsoo Park] Type coercion for udf inputs

Author: Cheolsoo Park

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

@@ -680,7 +680,7 @@ object HiveTypeCoercion {
       // Skip nodes who's children have not been resolved yet.
       case e if !e.childrenResolved => e
 
-      case e: ExpectsInputTypes =>
+      case e: ExpectsInputTypes if (e.inputTypes.nonEmpty) =>
         val children: Seq[Expression] = e.children.zip(e.inputTypes).map { case (in, expected) =>
           // If we cannot do the implicit cast, just use the original input.
           implicitCast(in, expected).getOrElse(in)

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

@@ -24,8 +24,11 @@ import org.apache.spark.sql.types.DataType
  * User-defined function.
  * @param dataType  Return type of function.
  */
-case class ScalaUDF(function: AnyRef, dataType: DataType, children: Seq[Expression])
-  extends Expression {
+case class ScalaUDF(
+    function: AnyRef,
+    dataType: DataType,
+    children: Seq[Expression],
+    inputTypes: Seq[DataType] = Nil) extends Expression with ExpectsInputTypes {
 
   override def nullable: Boolean = true