use normal completions in ACs passed to CreateIteratorFromClosure when possible

[michaelficarra]

CreateIteratorFromClosure passes the AC to GeneratorStart which treats any normal completion return value the same as a ReturnCompletion(*undefined*). It seems better to me to use NormalCompletion(~unused~) in these locations and only return a return completion if we want to use a value other than *undefined*.

[bakkot]

I don't like that we have this degree of freedom. We should pick one and stick with it, and ensure the machinery only supports that kind. I'll prepare some PRs so we can see what the machinery would look like in each case.

[bakkot]

PRs: https://github.com/tc39/ecma262/pull/3605 / https://github.com/tc39/ecma262/pull/3606.

I didn't do the actual changes to the ACs; that can wait until we decide which way we want to go. My preference is https://github.com/tc39/ecma262/pull/3606.