Improve parsing in await and yield context

[Kingwl]

Fixes #44459

It works. But I don't know why 🤷‍♂️.

[sandersn]

I need to run through it in the debugger to understand it myself, but my guess is that isBindingIdentifier accepts await and yield as keywords where isIdentifier doesn't (when not in an await/yeild context). Then something in parseVarExpression explicitly rejects await/yield. Not sure why it incorrectly works today.

[sandersn]

Well, I was sort of right. It's actually an error in the binder, from checkContextualIdentifier, not parseVarExpression. A better fix would be to change the definition of inYieldContext and inAwaitContext in isIdentifier to skip let await/let yield as well.

[sandersn]

Now that I've checked how to clear Yield/AwaitContext, I see that isBindingIdentifier is the same as isIdentifier without the special cases for yield/await.

I'd like to see two things:

1. Check whether the special case for yield/await is even needed. Try deleting it from isIdentifier and report back how baselines change. It could be that the binder has better error messages in all cases.
2. If that doesn't work out, put a small comment above isBindingIdentifier saying // let await/let yield are allowed here and disallowed in the binder (or something similar)

I think it's probably fine to call isBindingIdentifier instead of isIdentifier; isIdentifier hasn't changed for 5 years, and I assume @rbuckton created a isBindingIdentifier last year instead of passing a boolean to isIdentifier for performance reasons.

[sandersn]

See comments above.

[Kingwl]

Moved to #44680.