[SPARK-9151][SQL] Implement code generation for Abs

JIRA: https://issues.apache.org/jira/browse/SPARK-9151

Add codegen support for `Abs`.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #7498 from viirya/abs_codegen and squashes the following commits:

0c8410f [Liang-Chi Hsieh] Implement code generation for Abs.

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

@@ -73,6 +73,13 @@ case class Abs(child: Expression) extends UnaryExpression with ExpectsInputTypes
 
   private lazy val numeric = TypeUtils.getNumeric(dataType)
 
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String = dataType match {
+    case dt: DecimalType =>
+      defineCodeGen(ctx, ev, c => s"$c.abs()")
+    case dt: NumericType =>
+      defineCodeGen(ctx, ev, c => s"(${ctx.javaType(dt)})(java.lang.Math.abs($c))")
+  }
+
   protected override def nullSafeEval(input: Any): Any = numeric.abs(input)
 }
 

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala

@@ -278,6 +278,8 @@ final class Decimal extends Ordered[Decimal] with Serializable {
       Decimal(-longVal, precision, scale)
     }
   }
+
+  def abs: Decimal = if (this.compare(Decimal(0)) < 0) this.unary_- else this
 }
 
 object Decimal {