Ability to augment late bound properties

### Describe the bug

This:

```
model A {
prop: string;
other: string;
}

model B is OmitProperties;

@@doc(B.prop, "documentation");
```

Fails with `Model doesn't have member prop`.

I'm assuming it's something to do with the fact that `OmitProperties` creates an anonymous type and that that breaks the augmenting decorator but I may be wrong. Using a spread instead doesn't work.

### Reproduction

https://typespec.io/playground/?c=bW9kZWwgQSB7DQogIHByb3A6IHN0cmluZzvEEW90aGVyyxJ9DQoNCsYzQiBpcyBPbWl0UHJvcGVydGllczxBLCAixTMiPjvEKkBAZG9jKEIuxFgsICJkb2N1bWVudGF0aW9uIik7&e=%40typespec%2Fopenapi3&options=%7B%7D

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.