Add the printout

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;

1. Code simplification:

Item_default_value handled all these values:
a. DEFAULT(field)
b. DEFAULT
c. IGNORE
and had various conditions to distinguish (a) from (b) and from (c).

Introducing a new abstract class Item_contextually_typed_value_specification,
to handle (b) and (c), so the hierarchy now looks as follows:

Item
  Item_result_field
    Item_ident
      Item_field
        Item_default_value                      - DEFAULT(field)
  Item_contextually_typed_value_specification
    Item_default_specification                  - DEFAULT
    Item_ignore_specification                   - IGNORE

2. Introducing a new virtual method is_evaluable_expression() to
determine if an Item is:
- a normal expression, so its val_xxx()/get_date() methods can be called
- or a just an expression substitute, whose value methods cannot be called.

3. Disallowing Items that are not evalualble expressions in table value
   constructors.

fix_fields_for_tvc() could call fix_fields() for Items that have
already been fixed before. Changing fix_fields() to fix_fields_if_needed().

We disable the MDEV-21189 test galera.galera_partition
because it times out.