codegen: attach comments to span: 0

* https://github.com/rolldown/rolldown/issues/5482

```
/*! 
 * INDEX.TS (ABOVE IMPORT)
 * THIS COMMENT IS NOT PRESERVED
 */

import { foo } from './foo.ts';
```

This legal comment is attached to the import statement, it gets removed when the import statement is removed. Rolldown need to move the comment to span: 0.