Make eval() and genCode() final

Author: JoshRosen

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

@@ -184,10 +184,10 @@ abstract class Expression extends TreeNode[Expression] {
  */
 trait Unevaluable extends Expression {
 
-  override def eval(input: InternalRow = null): Any =
+  final override def eval(input: InternalRow = null): Any =
     throw new UnsupportedOperationException(s"Cannot evaluate expression: $this")
 
-  override protected def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String =
+  final override protected def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String =
     throw new UnsupportedOperationException(s"Cannot evaluate expression: $this")
 }