This PR fixes bugs found in the [turso-
go](https://github.com/tursodatabase/turso-go) driver with UPSERT clause
earlier, where `Gorm` will (obviously) use Expr::Variable's as well as
use quotes for `Expr::Qualified` in the tail end of an UPSERT statement.
Example:
```sql
INSERT INTO users (a,b,c) VALUES (?,?,?) ON CONFLICT (`users`.`a`) DO UPDATE SET b = `excluded`.`b`, a = ?;
```
and previously we were not properly calling `rewrite_expr`, which was
not properly setting the anonymous `Expr::Variable` to `__param_N` named
parameter, so it would ignore it completely, then return the wrong # of
parameters.
Also, we didn't handle quoted "`excluded`.`x`", so it would panic in the
optimizer that Qualified should have been rewritten earlier.
Closes#3157
8c53d7f024