Add lint checks for unused loop labels
Previously no warning was generated when a loop label was written out but never used (i.e. in a `break` or `continue` statement):
```rust
fn main() {
'unused_label: loop {}
}
```
would compile without complaint.
This fix introduces `-W unused_loop_label`, which generates the following warning message for the above snippet:
```
warning: unused loop label
--> main.rs:2:5
|
2 | 'unused_label: loop {}
| ^^^^^^^^^^^^^
|
= note: #[warn(unused_loop_label)] on by default
```
Fixes: #50751.