Auto merge of #96296 - cjgillot:remove-label-lt-shadow, r=petrochenkov

Remove label/lifetime shadowing warnings

This PR removes some pre-1.0 shadowing warnings for labels and lifetimes.

The current behaviour of the compiler is to warn
* labels that shadow unrelated labels in the same function --> removed
```rust
'a: loop {}
'a: loop {} // STOP WARNING
```

* labels that shadow enclosing labels --> kept, but only if shadowing is hygienic
```rust
'a: loop {
  'a: loop {} // KEEP WARNING
}
```

* labels that shadow lifetime --> removed
```rust
fn foo<'a>() {
  'a: loop {} // STOP WARNING
}
```

* lifetimes that shadow labels --> removed
```rust
'a: loop {
  let b = Box::new(|x: &i8| *x) as Box<dyn for <'a> Fn(&'a i8) -> i8>; // STOP WARNING
}
```

* lifetimes that shadow lifetimes --> kept
```rust
fn foo<'a>() {
  let b = Box::new(|x: &i8| *x) as Box<dyn for <'a> Fn(&'a i8) -> i8>; // KEEP WARNING
}
```

Closes rust-lang/rust#31745.

-----

From `@petrochenkov` in rust-lang/rust#95781 (comment)
> I think we should remove these silly checks entirely.
> They were introduced long time ago in case some new language features appear and require this space.
> Now we have another mechanism for such language changes - editions, and if "lifetimes in expressions" or something like that needs to be introduced it could be introduced as an edition change.
> However, there was no plans to introduce anything like for years, so it's unlikely that even the edition mechanism will be necessary.

r? rust-lang/lang

Author: bors