[SPARK-8285] [SQL] CombineSum should be calculated as unlimited decimal first

    case cs  CombineSum(expr) =>
        val calcType = expr.dataType
          expr.dataType match {
            case DecimalType.Fixed(_, _) =>
              DecimalType.Unlimited
            case _ =>
              expr.dataType
          }
calcType is always expr.dataType. credits are all belong to IntelliJ

Author: navis.ryu <navis@apache.org>

Closes #6736 from navis/SPARK-8285 and squashes the following commits:

20382c1 [navis.ryu] [SPARK-8285] [SQL] CombineSum should be calculated as unlimited decimal first