Suggest adding ' for incorrect loop labels

[varkor]

fn main() {
    label: loop {}
	//     ^^^^ error: expected identifier, found keyword `loop`
}

In situations like these, a more helpful suggestion would be to change label to 'label. I imagine this error could be special cased to look at the identifier and suggest something different for loop/while/for.

[ehuss]

This appears to be resolved by #81236.

error: malformed loop label
 --> src/main.rs:2:5
  |
2 |     label: loop {}
  |     ^^^^^ help: use the correct loop label format: `'label`