otherwise we'd need to store sql_mode *per vcol*
(consider CREATE INDEX...) and how SHOW CREATE TABLE would
support that?
Additionally, get rid of vcol::expr_str, just to make sure
the string is always generated and never leaked in the
original form.
Fix the doubly questional fix for MySQL Bug#17250787:
* it detected autoinc index by looking for the first index
that starts from autoinc column. never mind one column
can be part of many indexes.
* it used autoinc_field->field_index to look up into internal
innodb dictionary. But field_index accounts for virtual
columns too, while innodb dictionary ignores them.
Find the index by its name, like elsewhere in ha_innobase.
