[Bug]: Unable to make const declaration of union variant

### Describe the bug

A const declaration whose annotated type is a variant of a union cannot be assigned to.

This arises in the context of creating const declarations for discriminated unions where the discriminator field is a union.

### Reproduction

https://typespec.io/playground/?e=%40typespec%2Fopenapi3&c=bmFtZXNwYWNlIEV4YW1wbGU7DQoNCnVuaW9uIFNwZWNpZXMgew0KICAgIGNhdDogImNhdCINCn3EJi8vIEVycm9yOiBUeXBlICfFHCcgaXMgbm90IGFzc2lnbmFibGUgdG8gdMsixC7EXCh1bsolKQ0KY29uc3QgZeYAiFZhbHVlOugAhC5jYXQgPcwOOw%3D%3D&options=%7B%7D&vs=%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.