[SPARK-1812] sql/catalyst - Provide explicit type information

For Scala 2.11 compatibility.

Without the explicit type specification, withNullability
return type is inferred to be Attribute, and thus calling
at() on the returned object fails in these tests:

[ERROR] /Users/avati/work/spark/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala:370: value at is not a
[ERROR]     val c4_notNull = 'a.boolean.notNull.at(3)
[ERROR]                                         ^
[ERROR] /Users/avati/work/spark/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala:371: value at is not a
[ERROR]     val c5_notNull = 'a.boolean.notNull.at(4)
[ERROR]                                         ^
[ERROR] /Users/avati/work/spark/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala:372: value at is not a
[ERROR]     val c6_notNull = 'a.boolean.notNull.at(5)
[ERROR]                                         ^
[ERROR] /Users/avati/work/spark/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala:558: value at is not a
[ERROR]     val s_notNull = 'a.string.notNull.at(0)

Signed-off-by: Anand Avati <avatiredhat.com>

Author: Anand Avati <avati@redhat.com>

Closes #1709 from avati/SPARK-1812-notnull and squashes the following commits:

0470eb3 [Anand Avati] SPARK-1812: sql/catalyst - Provide explicit type information

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

@@ -134,7 +134,7 @@ case class AttributeReference(name: String, dataType: DataType, nullable: Boolea
   /**
    * Returns a copy of this [[AttributeReference]] with changed nullability.
    */
-  override def withNullability(newNullability: Boolean) = {
+  override def withNullability(newNullability: Boolean): AttributeReference = {
     if (nullable == newNullability) {
       this
     } else {