Outdated error on signature returning reference without reference arguments

[dtolnay]

The message "functions that return a reference must take exactly one input reference" was accurate only until #608. Now that a signature can be annotated with explicit lifetimes, either a 'static output lifetime would also be fine, or multiple input lifetimes would also be fine.

Current behavior:

#[cxx::bridge]
mod ffi {
    extern "C++" {
        unsafe fn f() -> &i32;
    }
}

error[cxxbridge]: functions that return a reference must take exactly one input reference
  ┌─ src/main.rs:4:16
  │
4 │         unsafe fn f() -> &i32;
  │                ^^^^^^^^^^^^^^^ functions that return a reference must take exactly one input reference