Fix parsing type casted generic flow arrow exprs

[existentialism]

Q	A
Fixed Issues?	Fixes #11716
Patch: Bug Fix?	Y
License	MIT

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[JLHwung]

While reviewing this PR, I realize there are two invalid cases related to how we handle parenItem and generic arrow:

<T> async () => {}

should throw but it is passed because we didn't validate arrowExpression.async.

(<T>() => {}?:any);

should throw but ? is incorrectly eaten in parseParenItem.

A regression was introduced.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/27618/

[nicolo-ribaudo]

Can you add the same tests but with enableParenthesizedExpressions?