[SameObject]/[NewObject] should be allowed on unions and other objects

[kainino0x]

This is extracted from #71 which discusses extending [SameObject]/[NewObject] to types other than Promises:

It should be allowed if the return value is any object type. Specifically, any interface type, "object", a promise, an Error, a DOMException, any buffer source type, or a union of types on which [NewObject] is allowed.

WebGPU in particular wants to use this on a readonly attribute with a union type. gpuweb/gpuweb#1225