(a change adding a helper method Column_definition::make_field())
to take into account the EXECUTE stage of a prepared statement.
This DBUG_ASSERT() caused crashes in a few tests when running "mtr --ps".

Part6: Deriving Item_type_holder from Type_handler_hybrid_real_field_type

my_free() can be called for C++ static destructors after
exit(), i.e after the THR_THD used by callback is already destroyed.

Part5: Derive Item_param from Type_handler_hybrid_field_type

A cleanup: removing members and functions that become unused:
- Item_func_hybrid_field_type::cached_result_type
- sp_map_result_type()

Part4: Deriving Item_temporal_hybrid_func from Type_handler_hybrid_field_type

Part3: Deriving Item_splocal from Type_handler_hybrid_field_type

Part2: deriving Item_sum_sum from Type_handler_hybrid_field_type,
removing "Item_result Item_sum_sum::hybrid_type".

(A dependency task for MDEV-4912 Add a plugin to field types)

Adding Column_definition::make_field() as a convenience
wrapper for ::make_field.

Added --version=str (optional argument sets server version string).

sp_variable::field_def.type, so there was data redundancy.

sp_head::fill_field_definition().

mysqlbug has been obsolete since MySQL 5.5, and has been removed in MySQL 5.7.
It's also of no use for reporting MariaDB bugs. The script and the associated
man page removed from MariaDB.

They aren't faster than normal mutexes. They're disabled by default for years,
so de facto it's dead code, never used.

            to match MyISAM

Renamed aria_recover to aria_recover_options.

The following left in semi-improved state to keep patch size reasonable:
- Field operator new: left thd_alloc(current_thd)
- Sql_alloc operator new: left thd_alloc(thd_get_current_thd())
- Item_args constructors: left thd_alloc(thd)
- Item_func_interval::fix_length_and_dec(): no THD arg, have to call current_thd
- Item_func_dyncol_exists::val_int(): same
- Item_dyncol_get::val_str(): same
- Item_dyncol_get::val_int(): same
- Item_dyncol_get::val_real(): same
- Item_dyncol_get::val_decimal(): same
- Item_singlerow_subselect::fix_length_and_dec(): same

Part2: moving a few other fields from Column_definition to Create_field.
sizeof(sp_variable) is now 200 bytes (vs 248 bytes in 10.1)

- MDEV-8093 sql_yacc.yy: add %type create_field for field_spec and column_def

and partially:

- MDEV-8095 Split Create_field

and don't set Create_field->sql_type to MYSQL_TYPE_VIRTUAL
temporarily only to change it again few lines later.

see commit 81e4ce5e

A prerequisite change for:
- MDEV-8093 sql_yacc.yy: add %type create_field for field_spec and column_def
- MDEV-8094 sql_yacc.yy: get rid of the rules "opt_if_not_exists_table_element"
            and "opt_if_exists_table_element"
- MDEV-8095 Split Create_field

(that is MY_CHECK_C_COMPILER_FLAG, MY_CHECK_CXX_COMPILER_FLAG,
MY_CHECK_AND_SET_COMPILER_FLAG)

This is to have a clear "internal" name prefix and to avoid
mixing them with hand-named symbols

A preparatory task for:
  MDEV-4912 Add a plugin to field types (column types)