MDEV-14347 CREATE PROCEDURE returns no error when using an unknown variable

CREATE PROCEDURE did not detect unknown SP variables in assignments like this:

  SET var=a_long_var_name_with_a_typo;

The error happened only during the SP execution time, and only of the control
flow reaches the erroneous statement.

Fixing most expressions to detect unknown identifiers.
This includes simple subqueries without tables:

- Query specification: SELECT list, WHERE,
  HAVING (inside aggregate functions) clauses, e.g.
    SET var= (SELECT unknown_ident+1);
    SET var= (SELECT 1 WHERE unknown_identifier);
    SET var= (SELECT 1 HAVING SUM(unknown_identifier);

- Table value constructor: VALUES clause, e.g.:
    SET var= (VALUES(unknown_ident));

Note, in some more complex subquery cases unknown variables are still not detected
(this will be fixed separately):

- Derived tables:
  SET a=(SELECT unknown_ident FROM (SELECT 1 AS alias) t1);
  SET res=(SELECT * FROM t1 LEFT OUTER JOIN (SELECT unknown_ident) t2 USING (c1));

- CTE:
  SET a=(WITH cte1 (a) AS (SELECT unknown_ident) SELECT * FROM cte1);
  SET a=(WITH cte1 (a,b) AS (VALUES (unknown,2),(3,4)) SELECT * FROM cte1);
  SET a=(WITH cte1 (a,b) AS (VALUES (1,2),(3,4)) SELECT unknown_ident FROM cte1);

- SELECT .. GROUP BY unknown_identifier
- SELECT .. ORDER BY unknown_identifier
- HAVING with an unknown identifier outside of any aggregate functions:
  SELECT .. HAVING unknown_identifier;

sql/sql_tvc.h

@@ -18,6 +18,7 @@
 #include "sql_type.h"
 
 typedef List<Item> List_item;
+typedef bool (Item::*Item_processor) (void *arg);
 class select_result;
 class Explain_select;
 class Explain_query;
@@ -65,6 +66,7 @@ public:
   bool exec(SELECT_LEX *sl);
 
   void print(THD *thd_arg, String *str, enum_query_type query_type);
+  bool walk_values(Item_processor processor, bool walk_subquery, void *arg);
 };
 
 st_select_lex *wrap_tvc_with_tail(THD *thd, st_select_lex *tvc_sl);