[SPARK-1938] [SQL] ApproxCountDistinctMergeFunction should return Int value.

`ApproxCountDistinctMergeFunction` should return `Int` value because the `dataType` of `ApproxCountDistinct` is `IntegerType`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #893 from ueshin/issues/SPARK-1938 and squashes the following commits:

3970e88 [Takuya UESHIN] Remove a superfluous line.
5ad7ec1 [Takuya UESHIN] Make dataType for each of CountDistinct, ApproxCountDistinctMerge and ApproxCountDistinct LongType.
cbe7c71 [Takuya UESHIN] Revert a change.
fc3ac0f [Takuya UESHIN] Fix evaluated value type of ApproxCountDistinctMergeFunction to Int.

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

@@ -82,7 +82,6 @@ abstract class AggregateFunction
   override def dataType = base.dataType
 
   def update(input: Row): Unit
-  override def eval(input: Row): Any
 
   // Do we really need this?
   override def newInstance() = makeCopy(productIterator.map { case a: AnyRef => a }.toArray)
@@ -166,7 +165,7 @@ case class CountDistinct(expressions: Seq[Expression]) extends AggregateExpressi
   override def children = expressions
   override def references = expressions.flatMap(_.references).toSet
   override def nullable = false
-  override def dataType = IntegerType
+  override def dataType = LongType
   override def toString = s"COUNT(DISTINCT ${expressions.mkString(",")})"
   override def newInstance() = new CountDistinctFunction(expressions, this)
 }
@@ -184,7 +183,7 @@ case class ApproxCountDistinctMerge(child: Expression, relativeSD: Double)
   extends AggregateExpression with trees.UnaryNode[Expression] {
   override def references = child.references
   override def nullable = false
-  override def dataType = IntegerType
+  override def dataType = LongType
   override def toString = s"APPROXIMATE COUNT(DISTINCT $child)"
   override def newInstance() = new ApproxCountDistinctMergeFunction(child, this, relativeSD)
 }
@@ -193,7 +192,7 @@ case class ApproxCountDistinct(child: Expression, relativeSD: Double = 0.05)
   extends PartialAggregate with trees.UnaryNode[Expression] {
   override def references = child.references
   override def nullable = false
-  override def dataType = IntegerType
+  override def dataType = LongType
   override def toString = s"APPROXIMATE COUNT(DISTINCT $child)"
 
   override def asPartial: SplitEvaluation = {
@@ -394,7 +393,7 @@ case class CountDistinctFunction(expr: Seq[Expression], base: AggregateExpressio
     }
   }
 
-  override def eval(input: Row): Any = seen.size
+  override def eval(input: Row): Any = seen.size.toLong
 }
 
 case class FirstFunction(expr: Expression, base: AggregateExpression) extends AggregateFunction {