There are two problems:

1. Field_typed_array field creates internal field for data
   conversion in Field_typed_array::init() and doesn't set
   convertion field index correctly while creating the field
   for temporary table.
Fixed by introducing new field method Field::set_field_index()
which updates convertion field index.

2. Convertion field and corresponding Item_field are created
   in TABLE::mem_root.
Fixed by replacing TABLE::mem_root with TABLE_SHARE's mem_root
for internal temporary tables.

Additional changes:

Public Field::field_index is replaced with private Field::m_field_index.
Added Field::field_index()/Field::set_field_index() methods.
Patch for 8.0 only.

Reviewed-by: Guilhem Bichot <guilhem.bichot@oracle.com>