Support #![cxx::bridge] inner attribute

[dtolnay]

A common idiom in my codebase is:

// lib.rs

mod ffi;
/* ... */

// ffi.rs

pub use self::ffi::*;

#[cxx::bridge]
mod ffi {
    /* ... */
}

// whatever.rs

use crate::ffi::Whatever;
/* ... */

It would be convenient for the ffi.rs to be expressible as:

// ffi.rs

#![cxx::bridge]

/* the usual contents of `mod ffi {...}` */

For now this would be blocked on rust-lang/rust#54726.