[SPARK-5324][SQL] Results of describe can't be queried

Make below code works.
```
sql("DESCRIBE test").registerTempTable("describeTest")
sql("SELECT * FROM describeTest").collect()
```

Author: OopsOutOfMemory <victorshengli@126.com>
Author: Sheng, Li <OopsOutOfMemory@users.noreply.github.com>

Closes #4249 from OopsOutOfMemory/desc_query and squashes the following commits:

6fee13d [OopsOutOfMemory] up-to-date
e71430a [Sheng, Li] Update HiveOperatorQueryableSuite.scala
3ba1058 [OopsOutOfMemory] change to default argument
aac7226 [OopsOutOfMemory] Merge branch 'master' into desc_query
68eb6dd [OopsOutOfMemory] Merge branch 'desc_query' of github.com:OopsOutOfMemory/spark into desc_query
354ad71 [OopsOutOfMemory] query describe command
d541a35 [OopsOutOfMemory] refine test suite
e1da481 [OopsOutOfMemory] refine test suite
a780539 [OopsOutOfMemory] Merge branch 'desc_query' of github.com:OopsOutOfMemory/spark into desc_query
0015f82 [OopsOutOfMemory] code style
dd0aaef [OopsOutOfMemory] code style
c7d606d [OopsOutOfMemory] rename test suite
75f2342 [OopsOutOfMemory] refine code and test suite
f942c9b [OopsOutOfMemory] initial
11559ae [OopsOutOfMemory] code style
c5fdecf [OopsOutOfMemory] code style
aeaea5f [OopsOutOfMemory] rename test suite
ac2c3bb [OopsOutOfMemory] refine code and test suite
544573e [OopsOutOfMemory] initial

sql/core/src/main/scala/org/apache/spark/sql/sources/ddl.scala

@@ -296,11 +296,14 @@ private[sql] case class ResolvedDataSource(provider: Class[_], relation: BaseRel
 private[sql] case class DescribeCommand(
     table: LogicalPlan,
     isExtended: Boolean) extends Command {
-  override def output = Seq(
+  override val output = Seq(
     // Column names are based on Hive.
-    AttributeReference("col_name", StringType, nullable = false)(),
+    AttributeReference("col_name", StringType, nullable = false, 
-    AttributeReference("data_type", StringType, nullable = false)(),
+      new MetadataBuilder().putString("comment", "name of the column").build())(),
-    AttributeReference("comment", StringType, nullable = false)())
+    AttributeReference("data_type", StringType, nullable = false, 
+      new MetadataBuilder().putString("comment", "data type of the column").build())(),
+    AttributeReference("comment", StringType, nullable = false, 
+      new MetadataBuilder().putString("comment", "comment of the column").build())())
 }
 
 private[sql] case class CreateTableUsing(

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveQl.scala

@@ -497,15 +497,14 @@ https://cwiki.apache.org/confluence/display/Hive/Enhanced+Aggregation%2C+Cube%2C
                 // TODO: Actually, a user may mean tableName.columnName. Need to resolve this issue.
                 val tableIdent = extractTableIdent(nameParts.head)
                 DescribeCommand(
-                  UnresolvedRelation(tableIdent, None), extended.isDefined)
+                  UnresolvedRelation(tableIdent, None), isExtended = extended.isDefined)
               case Token(".", dbName :: tableName :: colName :: Nil) =>
                 // It is describing a column with the format like "describe db.table column".
                 NativePlaceholder
               case tableName =>
                 // It is describing a table with the format like "describe table".
                 DescribeCommand(
-                  UnresolvedRelation(Seq(tableName.getText), None),
+                  UnresolvedRelation(Seq(tableName.getText), None), isExtended = extended.isDefined)
-                  extended.isDefined)
             }
           }
           // All other cases.

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveOperatorQueryableSuite.scala

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.hive.execution
+
+import org.apache.spark.sql.{Row, QueryTest}
+import org.apache.spark.sql.hive.test.TestHive._
+
+/**
+ * A set of tests that validates commands can also be queried by like a table
+ */
+class HiveOperatorQueryableSuite extends QueryTest {
+  test("SPARK-5324 query result of describe command") {
+    loadTestTable("src")
+
+    // register a describe command to be a temp table
+    sql("desc src").registerTempTable("mydesc")
+    checkAnswer(
+      sql("desc mydesc"),
+      Seq(
+        Row("col_name", "string", "name of the column"),
+        Row("data_type", "string", "data type of the column"),
+        Row("comment", "string", "comment of the column")))
+
+    checkAnswer(
+      sql("select * from mydesc"),
+      Seq(
+        Row("key", "int", null),
+        Row("value", "string", null)))
+
+    checkAnswer(
+      sql("select col_name, data_type, comment from mydesc"),
+      Seq(
+        Row("key", "int", null),
+        Row("value", "string", null)))
+  }
+}

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala

@@ -59,7 +59,7 @@ class HiveQuerySuite extends HiveComparisonTest with BeforeAndAfter {
     Locale.setDefault(originalLocale)
   }
 
-  test("SPARK-4908: concurent hive native commands") {
+  test("SPARK-4908: concurrent hive native commands") {
     (1 to 100).par.map { _ =>
       sql("USE default")
       sql("SHOW TABLES")