rustc_borrowck: fix async closure error note to report FnOnce rather than Fn

[InvalidPathException]

Fixes #148391

Root cause:
async closures are treated like plain Fn closures because we never updated their kind after adding the async coroutine path.

Symptom:
The compiler note tells users closure implements Fn…, which is wrong for async closures and made the error message confusing. Notably FnMut does not suffer from this issue.

Fix:
With this fix we treat coroutine closures as FnOnce when building that note.

Testing:
Updated a UI test stderr file to reflect the correct wording. It also clearly demonstrates what was wrong.

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[cjgillot]

Any way this can look like

let note = format!("closure implements `{}`, so references to captured variables can't escape the closure", closure_kind.as_str());

[InvalidPathException]

I did think about this. Correct me if I'm wrong, RegionNameSource stores these as &'static str but format! gives String, so we cannot really get around it (at least in this way). If we do refactor then it might be a lot of diff for something we do not touch for years.