A cleanup for MDEV-9217 Split Item::tmp_table_field_from_field_type() into virtual methods in Type_handler

Fixing that the server tried to create the old decimal for:

  CREATE TABLE t2 AS SELECT old_decimal_field FROM t1
               UNION SELECT bigint_field FROM t1;

  CREATE TABLE t2 AS SELECT old_decimal_field FROM t1
               UNION SELECT mediumint_field FROM t1;

  CREATE TABLE t2 AS SELECT year FROM t1
               UNION SELECT old_decimal_field FROM t1;

  CREATE TABLE t2 AS SELCT COALESCE(old_decimal_field) FROM t1;

Solution:

1. field_types_merge_rules[][] had three MYSQL_TYPE_DECIMAL remainders.
   Fixing to MYSQL_TYPE_NEWDECIMAL, like it is done for all other type pairs.
   This fixes the above queries with UNION.

2. Convert MYSQL_TYPE_DECIMAL to MYSQL_TYPE_NEWDECIMAL in
   Item::tmp_table_field_from_field_type().
   This fixed the above query with COALESCE.

3. Adding "new Field_decimal" into Type_handler_olddecimal::make_table_field().
   In case if something goes wrong it will crash on assert only in debug builds,
   while create the old decimal in release.
   Note, this "new Field_decimal" will be needed later anyway,
   when we reuse Type_handler::make_table_field() in make_field() in field.cc.