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.

1 files changed, 2 insertions, 2 deletions

diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 14d71139dad..edcd2171721 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -13673,13 +13673,13 @@ expr_or_default:
           expr { $$= $1;}
         | DEFAULT
           {
-            $$= new (thd->mem_root) Item_default_value(thd, Lex->current_context());
+            $$= new (thd->mem_root) Item_default_specification(thd);
             if (unlikely($$ == NULL))
               MYSQL_YYABORT;
           }
         | IGNORE_SYM
           {
-            $$= new (thd->mem_root) Item_ignore_value(thd, Lex->current_context());
+            $$= new (thd->mem_root) Item_ignore_specification(thd);
             if (unlikely($$ == NULL))
               MYSQL_YYABORT;
           }