Rollup merge of #75857 - dtolnay:unsafe, r=nagisa

Syntactically permit unsafety on mods

Similar to https://github.com/rust-lang/rust/pull/66183; we will accept these constructs syntactically but reject with a semantic check after macro expansion if a proc macro hasn't replaced it with something else meaningful to Rust.

```rust
#[mymacro]
unsafe mod m {
    ...
}

#[mymacro]
unsafe extern "C++" {
    ...
}
```

The intention is that this might be used as a kind of "item-level unsafe" in attribute macro DSLs -- holding things which are unsafe to declare but potentially safe to use. For example I look forward to using this in https://github.com/dtolnay/cxx.

In the absence of a procedural macro rewriting them to something else, they'll continue to be rejected at compile time though with a better error message than before.

### Before:

```console
error: expected item, found keyword `unsafe`
 --> src/main.rs:1:1
  |
1 | unsafe mod m {
  | ^^^^^^ expected item
```

### After:

```console
error: module cannot be declared unsafe
 --> src/main.rs:1:1
  |
1 | unsafe mod m {
  | ^^^^^^

error: extern block cannot be declared unsafe
 --> src/main.rs:4:1
  |
4 | unsafe extern "C++" {
  | ^^^^^^
```

Closes #68048.

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