[compiler] Allow `@discriminated` union variants to be unions when `envelope` is "none"

[witemple-msft]

Clear and concise description of the problem

Currently, in @discriminated(#{ envelope: "none"}) unions, we require variants to be models that have the discriminator property on them set to the correct value (Error: invalid-discriminated-union-variant if not). I propose that we allow a discriminated union variant to also be a union if each variant of the sub-union conforms to the discriminator property rule. This would allow constructs like the following:

@discriminated(#{ envelope: "none", discriminatorPropertyName: "kind" })
union X {
  a: A;
  b: B;
}

model A {
  kind: "a";
  //...
}

@discriminated(#{ envelope: "none", discriminatorPropertyName: "bKind" })
union B {
  c: C;
  d: D;
}

model C {
  kind: "b";
  bKind: "c";
  // ...
}

model D {
  kind: "b";
  bKind: "d";
  // ...
}

I ran into this trying to describe a schema with this sort of structure, where one of the variants of a top level union discriminated by "kind" itself has several subvariants discriminated by its own separate property.

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