Rollup merge of #127106 - spastorino:improve-unsafe-extern-blocks-diagnostics, r=compiler-errors

Improve unsafe extern blocks diagnostics

Closes #126327

For this code:

```rust
extern {
    pub fn foo();
    pub safe fn bar();
}
```

We get ...

```
error: items in unadorned `extern` blocks cannot have safety qualifiers
 --> test.rs:3:5
  |
3 |     pub safe fn bar();
  |     ^^^^^^^^^^^^^^^^^^
  |
help: add unsafe to this `extern` block
  |
1 | unsafe extern {
  | ++++++

error[E0658]: `unsafe extern {}` blocks and `safe` keyword are experimental
 --> test.rs:3:9
  |
3 |     pub safe fn bar();
  |         ^^^^
  |
  = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
  = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.
```

And then making the extern block unsafe, we get ...

```
error: extern block cannot be declared unsafe
 --> test.rs:1:1
  |
1 | unsafe extern {
  | ^^^^^^
  |
  = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
  = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable

error: items in unadorned `extern` blocks cannot have safety qualifiers
 --> test.rs:3:5
  |
3 |     pub safe fn bar();
  |     ^^^^^^^^^^^^^^^^^^

error[E0658]: `unsafe extern {}` blocks and `safe` keyword are experimental
 --> test.rs:3:9
  |
3 |     pub safe fn bar();
  |         ^^^^
  |
  = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
  = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0658`.
```

r? ``@compiler-errors``

0 files changed, 0 insertions, 0 deletions