[Bug]: named union with circular ref should not pass compile

### Describe the bug

union defined with any kinds of circular ref should be banned because it is meaningless.

### Reproduction

[playground](https://cadlplayground.z22.web.core.windows.net/?c=dW5pb24gVGVzdCB7DQogxQosDQp9DQoNCsYcQcYZQizEBkPOHELGHCIxIsUeIjLECMsgQ8YgQcYeM8ce&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.