Editorial: FunctionDeclarationInstantiation cannot return ReturnCompletion

[bakkot]

This is because the only way to get a return completion in expression position is with a yield, and yield is statically forbidden in generator formal parameters.

I suspect esmeta will complain about this, because FunctionDeclarationInstantiation calls IteratorBindingInitialization to do the initialization, and that AO is also used for let [foo] = bar declarations, which can contain yield expressions and therefore can produce return completions. I didn't think it was worth adding an assert in FunctionDeclarationInstantiation which says that IteratorBindingInitialization cannot produce a return completion in this specific case, but I'm open to doing so if we can do it without making the algorithm too awkward.

This imprecision is exposed by #3606 failing esmeta's typechecks. It should pass if rebased on this (I think).