[SPARK-2518][SQL] Fix foldability of Substring expression.

This is a follow-up of #1428.

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

Closes #1432 from ueshin/issues/SPARK-2518 and squashes the following commits:

37d1ace [Takuya UESHIN] Fix foldability of Substring expression.

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

@@ -216,6 +216,8 @@ case class Substring(str: Expression, pos: Expression, len: Expression) extends
   
   type EvaluatedType = Any
 
+  override def foldable = str.foldable && pos.foldable && len.foldable
+
   def nullable: Boolean = str.nullable || pos.nullable || len.nullable
   def dataType: DataType = {
     if (!resolved) {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/ConstantFoldingSuite.scala

@@ -201,7 +201,12 @@ class ConstantFoldingSuite extends PlanTest {
           Like(Literal(null, StringType), "abc") as 'c13,
           Like("abc", Literal(null, StringType)) as 'c14,
 
-          Upper(Literal(null, StringType)) as 'c15)
+          Upper(Literal(null, StringType)) as 'c15,
+
+          Substring(Literal(null, StringType), 0, 1) as 'c16,
+          Substring("abc", Literal(null, IntegerType), 1) as 'c17,
+          Substring("abc", 0, Literal(null, IntegerType)) as 'c18
+        )
 
     val optimized = Optimize(originalQuery.analyze)
 
@@ -228,8 +233,12 @@ class ConstantFoldingSuite extends PlanTest {
           Literal(null, BooleanType) as 'c13,
           Literal(null, BooleanType) as 'c14,
 
-          Literal(null, StringType) as 'c15)
-        .analyze
+          Literal(null, StringType) as 'c15,
+
+          Literal(null, StringType) as 'c16,
+          Literal(null, StringType) as 'c17,
+          Literal(null, StringType) as 'c18
+        ).analyze
 
     comparePlans(optimized, correctAnswer)
   }