MDEV-39119: Improve error handling when using OLD_VALUE as alias name

Analysis:
Since OLD_VALUE_SYM was part of reserved keywords, it did not allow
old_value in the alias.
Fix:
Change OLD_VALUE_SYM from reserved keyword to keyword_sp_var_and_label.

Author: mariadb-RuchaDeodhar

mysql-test/main/update.result

@@ -1128,4 +1128,13 @@ UPDATE t3 SET a = 10 RETURNING a;
 a
 10
 DROP TABLE t3;
+#
+# MDEV-39119: Improve error handling when using OLD_VALUE as alias name
+#
+CREATE OR REPLACE TABLE t3(a INT);
+INSERT INTO t3 VALUES (1);
+UPDATE t3 SET a = 10 RETURNING OLD_VALUE(a) AS old_value;
+old_value
+1
+DROP TABLE t3;
 # End of 13.0 test

mysql-test/main/update.test

@@ -985,4 +985,14 @@ UPDATE t3 SET a = 10 RETURNING a;
 
 DROP TABLE t3;
 
+--echo #
+--echo # MDEV-39119: Improve error handling when using OLD_VALUE as alias name
+--echo #
+
+CREATE OR REPLACE TABLE t3(a INT);
+INSERT INTO t3 VALUES (1);
+UPDATE t3 SET a = 10 RETURNING OLD_VALUE(a) AS old_value;
+
+DROP TABLE t3;
+
 --echo # End of 13.0 test

sql/sql_yacc.yy

@@ -17191,6 +17191,7 @@ keyword_sp_var_and_label:
         | MINUTE_SYM
         | MONTH_SYM
         | NEXTVAL_SYM
+        | OLD_VALUE_SYM
         | OVERLAPS_SYM
         | RECORD_SYM
 %ifdef MARIADB
@@ -17346,7 +17347,6 @@ reserved_keyword_udt_not_param_type:
         | NTILE_SYM
         | NULL_SYM
         | NTH_VALUE_SYM
-        | OLD_VALUE_SYM
         | ON
         | OPTIMIZE
         | OPTIONALLY