Account for type ascription when trying to label a loop

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Oh, whoops. I use a screen reader and I never have it set to announce
all punctuation so I forgot the apostrophes. Perhaps we can add a
suggestion inthe compiler if it sees that kind of pattern?

@ethindp
https://doc.rust-lang.org/stable/rust-by-example/flow_control/loop/nested.html:
func_loop: → 'func_loop:

Originally posted by @ethindp in #23416 (comment)

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Fixed in #81236.