Fix `unused_unsafe` label with `unsafe_block_in_unsafe_fn
Previously, the following code:
```rust
#![feature(unsafe_block_in_unsafe_fn)]
unsafe fn foo() {
unsafe { unsf() }
}
unsafe fn unsf() {}
```
Would give the following warning:
```
warning: unnecessary `unsafe` block
--> src/lib.rs:4:5
|
4 | unsafe { unsf() }
| ^^^^^^ unnecessary `unsafe` block
|
= note: `#[warn(unused_unsafe)]` on by default
```
which doesn't point out that the block is in an `unsafe fn`.
Tracking issue: #71668
cc #79208
47121d6d88