Auto merge of #125077 - spastorino:add-new-fnsafety-enum2, r=jackh726

Rename Unsafe to Safety

Alternative to #124455, which is to just have one Safety enum to use everywhere, this opens the posibility of adding `ast::Safety::Safe` that's useful for unsafe extern blocks.

This leaves us today with:

```rust
enum ast::Safety {
    Unsafe(Span),
    Default,
    // Safe (going to be added for unsafe extern blocks)
}

enum hir::Safety {
    Unsafe,
    Safe,
}
```

We would convert from `ast::Safety::Default` into the right Safety level according the context.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1712,7 +1712,7 @@ fn confirm_closure_candidate<'cx, 'tcx>(
                     [sig.tupled_inputs_ty],
                     output_ty,
                     sig.c_variadic,
-                    sig.unsafety,
+                    sig.safety,
                     sig.abi,
                 )
             })