Fix tests by binding attributes.

Author: JoshRosen

sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala

@@ -270,9 +270,10 @@ case class UnsafeExternalSort(
     assert(codegenEnabled, "UnsafeExternalSort requires code generation to be enabled")
     def doSort(iterator: Iterator[InternalRow]): Iterator[InternalRow] = {
       val ordering = newOrdering(sortOrder, child.output)
-      val prefixComparator = SortPrefixUtils.getPrefixComparator(sortOrder.head)
+      val boundSortExpression = BindReferences.bindReference(sortOrder.head, child.output)
+      val prefixComparator = SortPrefixUtils.getPrefixComparator(boundSortExpression)
       val prefixComputer = {
-        val prefixComputer = SortPrefixUtils.getPrefixComputer(sortOrder.head)
+        val prefixComputer = SortPrefixUtils.getPrefixComputer(boundSortExpression)
         new UnsafeExternalRowSorter.PrefixComputer {
           override def computePrefix(row: InternalRow): Long = prefixComputer(row)
         }

sql/core/src/test/scala/org/apache/spark/sql/execution/UnsafeExternalSortSuite.scala

@@ -26,7 +26,6 @@ import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.test.TestSQLContext
 import org.apache.spark.sql.types._
 
-
 class UnsafeExternalSortSuite extends SparkPlanTest with BeforeAndAfterAll {
 
   override def beforeAll(): Unit = {