It was a communication mismatch between sql layer and innodb.
The following Alter_inplace_info flags are split into a flag for
virtual columns and a flag for stored columns:
ALTER_ADD_COLUMN, ALTER_DROP_COLUMN, ALTER_COLUMN_TYPE,
ALTER_COLUMN_ORDER.
With this finer-grained information, InnoDB's logic is improved:
- more operations are in-place
- operations which should not be in-place are better caught.

Some flags from Alter_info are not needed anymore and removed:
ALTER_STORED_GCOLUMN, ALTER_VIRTUAL_GCOLUMN ; the former was part of
Alter_info which is intended for parser<->DDL communication; that flag
was used for communication between different functions of the server
in a DDL, which isn't the mission of Alter_info but of
Alter_table_ctx; it is replaced with a new
Alter_table_ctx::requires_generated_column_server_evaluation.

HA_ALTER_STORED_GCOL was also unneeded and removed.

ALTER_STORED_COLUMN_ORDER is set by a logic which ignores virtual
columns; indeed the creation/removal of those is not affecting the
format of rows. This allows in-place ALTER when adding/removing a
virtual column.