Parse `safe` items in extern blocks

[dtolnay]

Tracking issue: rust-lang/rust#123743
RFC: rust-lang/rfcs#3484

unsafe extern "C" {
    safe static TEST1: i32;
    safe fn test1(i: i32);
}

https://github.com/rust-lang/rust/blob/76e7a0849c07d73e4d9afde8036ee8c450127cc8/tests/ui/rust-2024/unsafe-extern-blocks/safe-items.rs