Editorial: have CreateIteratorFromClosure provide a new execution context to GeneratorStart

[bakkot]

See more complete discussion in https://es.discourse.group/t/execution-context-suspension-and-resumption/756. I'll summarize:

GeneratorStart manipulates the code evaluation state of the running execution context, and also stores that context on the generator object. Prior to #2045, the sole caller of GeneratorStart was in EvaluateGeneratorBody (although it wasn't called that at the time). So, in particular, GeneratorStart was only invoked in step 7 of [[Call]] for ordinary functions, at which time the running execution context would have been one created specifically for the evaluation of the generator in step 3 of [[Call]] - that is, the execution state being manipulated and stored would have been created specifically for the generator. And it would be removed from the execution context stack immediately after the call to GeneratorStart (in step 8 of [[Call]]) (and so implicitly suspended), so it makes sense to be describing what should happen after resumption at this point.

But #2045 introduced a new callsite for GeneratorStart, CreateIteratorFromClosure. Here the execution context is not created for the "generator"; it's just whatever happens to be running at the time. This doesn't really make sense - you don't want to be manipulating the execution state of the caller of CreateIteratorFromClosure!

The fix, I believe, is to have CreateIteratorFromClosure create a push a new context before calling GeneratorStart, and pop it after. That way it can "belong" to the synthetic generator it's creating. That is what this PR does.

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Note to self: this needs to address CreateAsyncIteratorFromClosure in the same way.

[bakkot]

I suppose an alternative is to have GeneratorStart create an execution context itself, and never push it to the execution context stack. That has the advantage that it would not be manipulating the code evaluation state of the running execution context, but the disadvantage that there would then be two execution contexts created for a generator: one for evaluating the parameters, created by [[Call]], and one for evaluating the body, created by GeneratorStart.

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