[SPARK-8283][SQL] Resolve udf_struct test failure in HiveCompatibilitySuite

This PR aimed to resolve udf_struct test failure in HiveCompatibilitySuite.

Currently, this is done by loosening CreateStruct's children type from NamedExpression to Expression and automatically generating StructField name for non-NamedExpression children.

The naming convention for unnamed children follows the udf's counterpart in Hive:
`col1, col2, col3, ...`

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #6828 from yijieshen/SPARK-8283 and squashes the following commits:

6052b73 [Yijie Shen] Doc fix
677e0b7 [Yijie Shen] Resolve udf_struct test failure by automatically generate structField name for non-NamedExpression children

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

@@ -53,7 +53,7 @@ case class CreateArray(children: Seq[Expression]) extends Expression {
  * Returns a Row containing the evaluation of all children expressions.
  * TODO: [[CreateStruct]] does not support codegen.
  */
-case class CreateStruct(children: Seq[NamedExpression]) extends Expression {
+case class CreateStruct(children: Seq[Expression]) extends Expression {
 
   override def foldable: Boolean = children.forall(_.foldable)
 
@@ -62,9 +62,14 @@ case class CreateStruct(children: Seq[NamedExpression]) extends Expression {
   override lazy val dataType: StructType = {
     assert(resolved,
       s"CreateStruct contains unresolvable children: ${children.filterNot(_.resolved)}.")
-    val fields = children.map { child =>
-      StructField(child.name, child.dataType, child.nullable, child.metadata)
-    }
+      val fields = children.zipWithIndex.map { case (child, idx) =>
+        child match {
+          case ne: NamedExpression =>
+            StructField(ne.name, ne.dataType, ne.nullable, ne.metadata)
+          case _ =>
+            StructField(s"col${idx + 1}", child.dataType, child.nullable, Metadata.empty)
+        }
+      }
     StructType(fields)
   }
 

sql/hive/compatibility/src/test/scala/org/apache/spark/sql/hive/execution/HiveCompatibilitySuite.scala

@@ -933,7 +933,7 @@ class HiveCompatibilitySuite extends HiveQueryFileTest with BeforeAndAfter {
     "udf_stddev_pop",
     "udf_stddev_samp",
     "udf_string",
-    // "udf_struct",  TODO: FIX THIS and enable it.
+    "udf_struct",
     "udf_substring",
     "udf_subtract",
     "udf_sum",