[SPARK-23979][SQL] MultiAlias should not be a CodegenFallback

## What changes were proposed in this pull request?

Just found `MultiAlias` is a `CodegenFallback`. It should not be as looks like `MultiAlias` won't be evaluated.

## How was this patch tested?

Existing tests.

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

Closes #21065 from viirya/multialias-without-codegenfallback.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/unresolved.scala

@@ -21,7 +21,7 @@ import org.apache.spark.sql.AnalysisException
 import org.apache.spark.sql.catalyst.{FunctionIdentifier, InternalRow, TableIdentifier}
 import org.apache.spark.sql.catalyst.errors.TreeNodeException
 import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, CodegenFallback, ExprCode}
+import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCode}
 import org.apache.spark.sql.catalyst.parser.ParserUtils
 import org.apache.spark.sql.catalyst.plans.logical.{LeafNode, LogicalPlan, UnaryNode}
 import org.apache.spark.sql.catalyst.trees.TreeNode
@@ -335,7 +335,7 @@ case class UnresolvedRegex(regexPattern: String, table: Option[String], caseSens
  * @param names the names to be associated with each output of computing [[child]].
  */
 case class MultiAlias(child: Expression, names: Seq[String])
-  extends UnaryExpression with NamedExpression with CodegenFallback {
+  extends UnaryExpression with NamedExpression with Unevaluable {
 
   override def name: String = throw new UnresolvedException(this, "name")