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.