Fix Async Generic After Await Parsing Error

[liamfd]

Q	A
Fixed Issues?	Fixes #11007
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Tests Added + Pass?	yes
Documentation PR Link	n/a
Any Dependency Changes?	no
License	MIT

As outlined in #11007, code like the following would fail to compile:

export const foo = async () => {
  await Promise.resolve({});

  const bar = async <T>() => 'foo';
};

This would only happen with an await followed by an async arrow function with a TypeScript generic defined. @JLHwung helped me understand the issue, and suggested the fix I've implemented in this PR.

In short, when parsing the snippet above, Babel would throw an Unexpected token, expected ";" error, which was caused by an Await cannot be used as name inside an async function (2:2) error. I've attempted to address this by resetting awaitPos in tsTryParseGenericAsyncArrowFunction.

Thanks in advance for any feedback, and thank you @JLHwung for your guidance on this issue!

[liamfd]

I wasn't 100% sure where to put this change, and the one below to reset things.

For resetting these values, it seemed best to do it before any attempt to parse is started, but strictly speaking it only seemed to need to happen before parseFunctionParams.

For setting state back to oldState below, it looks like typically this is done before parseArrowExpression is called (e.g. https://github.com/liamfd/babel/blob/master/packages/babel-parser/src/parser/expression.js#L972), so I matched that.

[liamfd]

I added oldMaybeInArrowParameters and oldYieldPos at @JLHwung 's suggestion. From looking around, it does seem like these are generally reset together.

I also wasn't entirely sure whether oldMaybeInArrowParameters should be set here, or whether it should be true or false. I opted to match https://github.com/liamfd/babel/blob/master/packages/babel-parser/src/parser/expression.js#L972, true seemed more appropriate.

[liamfd]

Looks like I've got some failures, I'll try to address them tonight.

[JLHwung]

The CI error is unrelated.

[nicolo-ribaudo]

Thanks!