Auto merge of #50763 - KyleStach1678:unused-loop-label, r=petrochenkov

Auto merge of #50763 - KyleStach1678:unused-loop-label, r=petrochenkov

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.