[heycam/webidl] Union types with colliding enum values (#219)

consider:

```webidl

enum A {
  "foo",
  "bar"
};

enum B {
  "foo",
  "baz"
};

interface C {
  attribute (A or B) prop;
};
```

what happens if we do `someC.prop = "foo"`?

I think probably we should just disallow union types where the flattened member types contain enums with conflicting values.

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