Allow mutable return reference on signatures with any lifetime in argument

[dtolnay]

As of #608 now that extern types defined in the bridge can be generic over lifetimes, it no longer makes sense to expect specifically a reference in one of the function arguments of a function that returns a mutable reference. The following two signatures would be perfectly valid:

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        type Thing<'a>;
    }

    unsafe extern "C++" {
        fn f<'a>(t: Box<Thing<'a>>) -> &'a mut i32;
        fn g(t: Box<Thing>) -> &mut i32;  // same thing, with lifetime elision
    }
}

pub struct Thing<'a>(&'a mut i32);

Current behavior:

error[cxxbridge]: &mut return type is not allowed unless there is a &mut argument
  ┌─ src/main.rs:8:9
  │
8 │         fn f<'a>(t: Box<Thing<'a>>) -> &'a mut i32;
  │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ &mut return type is not allowed unless there is a &mut argument

error[cxxbridge]: &mut return type is not allowed unless there is a &mut argument
  ┌─ src/main.rs:9:9
  │
9 │         fn g(t: Box<Thing>) -> &mut i32;
  │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ &mut return type is not allowed unless there is a &mut argument