[SPARK-21338][SQL] implement isCascadingTruncateTable() method in AggregatedDialect

## What changes were proposed in this pull request?

org.apache.spark.sql.jdbc.JdbcDialect's method:
def isCascadingTruncateTable(): Option[Boolean] = None
is not overriden in org.apache.spark.sql.jdbc.AggregatedDialect class.
Because of this issue, when you add more than one dialect Spark doesn't truncate table because isCascadingTruncateTable always returns default None for Aggregated Dialect.
Will implement isCascadingTruncateTable in AggregatedDialect class in this PR.

## How was this patch tested?

In JDBCSuite, inside test("Aggregated dialects"), will add one line to test AggregatedDialect.isCascadingTruncateTable

Author: Huaxin Gao <[email protected]>

Closes apache#19256 from huaxingao/spark-21338.

Author: huaxingao

sql/core/src/main/scala/org/apache/spark/sql/jdbc/AggregatedDialect.scala

@@ -41,4 +41,8 @@ private class AggregatedDialect(dialects: List[JdbcDialect]) extends JdbcDialect
   override def getJDBCType(dt: DataType): Option[JdbcType] = {
     dialects.flatMap(_.getJDBCType(dt)).headOption
   }
+
+  override def isCascadingTruncateTable(): Option[Boolean] = {
+    dialects.flatMap(_.isCascadingTruncateTable()).reduceOption(_ || _)
+  }
 }

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -740,11 +740,13 @@ class JDBCSuite extends SparkFunSuite
         } else {
           None
         }
+      override def isCascadingTruncateTable(): Option[Boolean] = Some(true)
     }, testH2Dialect))
     assert(agg.canHandle("jdbc:h2:xxx"))
     assert(!agg.canHandle("jdbc:h2"))
     assert(agg.getCatalystType(0, "", 1, null) === Some(LongType))
     assert(agg.getCatalystType(1, "", 1, null) === Some(StringType))
+    assert(agg.isCascadingTruncateTable() === Some(true))
   }
 
   test("DB2Dialect type mapping") {