MDEV-16309 Split ::create_tmp_field() into virtual methods in Item

Detailed: changes:
1. Moving Field specific code into new methods on Field:
   - Field *Field::create_tmp_field(...)
   - virtual void init_for_tmp_table(...)

2. Removing virtual Item::create_tmp_field().
   Adding instead a new virtual method Item::create_tmp_field_ex().

   Note, a virtual create_tmp_field() still exists, but only for Item_sum.
   This resembles 10.0 code structure. Perhaps create_tmp_field() should
   be removed from Item_sum and Item_sum descendants should override
   create_tmp_field_ex() directly. This can be done in a separate commit.

3. Adding helper classes Tmp_field_src and Tmp_field_param,
   to make the API for Item::create_tmp_field_ex() smaller
   and easier to extend in the future.

4. Decomposing the public function create_tmp_field() into
   virtual implementations for Item and a number of its descendants:
   - Item_basic_value
   - Item_sp_variable
   - Item_name_const
   - Item_result_field
   - Item_field
   - Item_ref
   - Item_type_holder
   - Item_row
   - Item_func_sp
   - Item_func_user_var
   - Item_sum
   - Item_sum_field
   - Item_proc

5. Adding DBUG_ASSERT-only virtual implementations for
   Item types that should not appear in create_tmp_table_ex(),
   for easier debugging:
   - Item_nodeset_func
   - Item_nodeset_to_const_comparator
   - Item_null_result
   - Item_copy
   - Item_ident_for_show
   - Item_user_var_as_out_param

6. Moving public function create_tmp_field_from_field()
   as a method to Item_field.

7. Removing Item::set_result_field(). It's not needed any more.

8. Cleanup: Removing the enum value "EXPR_CACHE_ITEM",
   as it's not used for a very long time.

sql/sql_select.h

@@ -1804,10 +1804,6 @@ bool setup_copy_fields(THD *thd, TMP_TABLE_PARAM *param,
 void copy_fields(TMP_TABLE_PARAM *param);
 bool copy_funcs(Item **func_ptr, const THD *thd);
 uint find_shortest_key(TABLE *table, const key_map *usable_keys);
-Field* create_tmp_field_from_field(THD *thd, Field* org_field,
-                                   LEX_CSTRING *name, TABLE *table,
-                                   Item_field *item);
-
 bool is_indexed_agg_distinct(JOIN *join, List<Item_field> *out_args);
 
 /* functions from opt_sum.cc */
@@ -2059,12 +2055,6 @@ bool mysql_select(THD *thd,
 void free_underlaid_joins(THD *thd, SELECT_LEX *select);
 bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit,
                          select_result *result);
-Field *create_tmp_field(THD *thd, TABLE *table,Item *item, Item::Type type,
-			Item ***copy_func, Field **from_field,
-                        Field **def_field,
-			bool group, bool modify_item,
-			bool table_cant_handle_bit_fields,
-                        bool make_copy_field);
 
 /*
   General routine to change field->ptr of a NULL-terminated array of Field