MDEV-21995 Server crashes in Item_field::real_type_handler with table value constructor

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.

mysql-test/main/table_value_constr.test

@@ -1339,3 +1339,17 @@ BEGIN NOT ATOMIC
 END;
 $$
 DELIMITER ;$$
+
+
+--echo #
+--echo # MDEV-21995 Server crashes in Item_field::real_type_handler with table value constructor
+--echo #
+
+--error ER_UNKNOWN_ERROR
+VALUES (IGNORE);
+--error ER_UNKNOWN_ERROR
+VALUES (DEFAULT);
+--error ER_UNKNOWN_ERROR
+EXECUTE IMMEDIATE 'VALUES (?)' USING IGNORE;
+--error ER_UNKNOWN_ERROR
+EXECUTE IMMEDIATE 'VALUES (?)' USING DEFAULT;