Generated columns that reference a later generated column return `NULL`

MySQL allows one generated column 's formula to reference another generated column, but only if that other column is defined *earlier* in the table. If defined *later* the table creation is rejected.

Dolt does not enforce the ordering rule. It accepts a `CREATE TABLE` and computes the offending column as `NULL`.

```sql
-- Dolt (v2.2.1)
CREATE TABLE fwd_gen (
a INT,
b INT AS (c + 1) VIRTUAL, -- references c, defined later
c INT AS (a + 1) VIRTUAL
);
INSERT INTO fwd_gen (a) VALUES (10);
SELECT a, b, c FROM fwd_gen;
+----+------+----+
| a | b | c |
+----+------+----+
| 10 | NULL | 11 |
+----+------+----+
1 row in set (0.00 sec)
```
```sql
-- MySQL (8.4.8)
CREATE TABLE fwd_gen (
a INT,
b INT AS (c + 1) VIRTUAL,
c INT AS (a + 1) VIRTUAL
);
-- ERROR 3107 (HY000): Generated column can refer only to generated columns defined prior to it.
```
`CREATE TABLE` and `ALTER TABLE` should reject the above shape.

Reproduce the CREATE TABLE example and then check the corresponding ALTER TABLE case. Trace the generated-column validation for both entry points and ensure a generated column cannot reference a later generated column. Done means both statements reject the forward reference instead of creating a column that evaluates to NULL.

Written by the indexing model from the issue text.