Incompatibilities: ALTER COLUMN with generated columns is not supported

[apstndb]

This incompatibility is detected by #90.

Spec

It is not explicitly written in DDL reference, but it is supported with/without NOT NULL.
https://cloud.google.com/spanner/docs/generated-column/how-to#modify-generated-column

Reproducing input

ALTER TABLE T ALTER COLUMN expr INT64 NOT NULL AS (1) STORED;

Current error

syntax error:testdata/generated_column.test:6:48: expected token: <eof>, but: AS

  6:  ALTER TABLE T ALTER COLUMN expr INT64 NOT NULL AS (1) STORED;
                                                     ^~