Invalid OA3 for unions

We are not emitting proper OA3 for unions. In [this example](https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAY2FkbC1sYW5nL3Jlc3QiOw0KdXNpbmcgQ2FkbC5SZXN0Ow0KDQpAZGlzY3JpbWluYXRvcigia2luZCIpDQptb2RlbCBCYXNlIHsNCn3EK8YTQSBleHRlbmRzyR0gIMQ1OiAiQcRlyytC2itCyStvcCBmb28oKTrFJTs%3D), I believe we need to emit `oneOf` somewhere. Right now we depend on references to the base type implicitly being a reference to some kind of union of anything that allOf's that type, but these semantics are not supported by the specification. @mikekistler agrees with this analysis.

oneOf two things need to happen for out output to be correct:

1. Any references to `Base` are replaced by an inline `oneOf` union referencing all of the derived types.
2. `Base` becomes an allOf of the derived types and we generate a new type with a name like `BaseCommon` that holds the cadl `Base` type's properties.

The latter is what @mikekistler prefers, and hinges on the fact that by putting discriminator on the base model you're essentially instructing cadl that "every time I reference this type, I'm referring to a union of its subtypes", but it does have the downside that we are generating a name in the openapi output (BaseCommon), and also munging a name users might expect to be generated as-is in client code (e.g. they might expect to see `class A extends Base { }` in their TS/C# code somewhere.

We have a third option here which is to delete @discriminator on models and reserve it only for unions ala #335 where the Cadl author gives a good name to both the base type and the union separately.