Auto merge of #51697 - estebank:once-used-lifetime-label, r=oli-obk

Add label to lint for lifetimes used once

```
error: lifetime parameter `'a` only used once
  --> $DIR/fn-types.rs:19:10
   |
LL |   a: for<'a> fn(&'a u32), //~ ERROR `'a` only used once
   |          ^^      -- ...is used only here
   |          |
   |          this lifetime...
```