Nonlocal extern "Rust" types trick in #942 doesn't work with Boxed types

[djmitche]

I tried the trick in #942 to use an opaque type in multiple Rust modules no longer works.

// task.rs
#[cxx::bridge(namespace = "tc")]
mod ffi {
    extern "Rust" {
        type Task;
    }
}

pub struct Task(tc::Task);
unsafe impl cxx::ExternType for Task {
    type Id = cxx::type_id!("tc::Task");
    type Kind = cxx::kind::Opaque;
}

// replica.rs
#[cxx::bridge(namespace = "tc")]
mod ffi {
    extern "C++" {
        include!("taskchampion-cpp/task.h");
        type Task = crate::task::Task;
    }
    extern "Rust" {
        fn foo(task: Box<Task>);
    }
}
use crate::task::Task;
fn foo(task: Box<Task>) { todo!() }

This results in

error[E0271]: type mismatch resolving `<Task as ExternType>::Kind == Trivial`
   --> src/tc/src/replica.rs:11:14
    |
11  |         type Task = crate::task::Task;
    |              ^^^^ type mismatch resolving `<Task as ExternType>::Kind == Trivial`
    |
note: expected this to be `Trivial`
   --> src/tc/src/task.rs:14:17
    |
14  |     type Kind = cxx::kind::Opaque;
    |                 ^^^^^^^^^^^^^^^^^
note: required by a bound in `verify_extern_kind`
   --> /home/dustin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cxx-1.0.119/src/extern_type.rs:187:41
    |
187 | pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind: self::Kind>() {}
    |                                         ^^^^^^^^^^^ required by this bound in `verify_extern_kind`

Changing the type to Trivial in the ExternType impl results in

/home/dustin/p/taskwarrior/build/src/tc/corrosion_generated/cxxbridge/taskchampion-cpp/src/replica.cpp:457:40: error: static assertion failed: type tc::Task should be trivially move constructible and trivially destructible in C++ to be used as type Box<Task> in Rust
  457 |     ::rust::IsRelocatable<::tc::Task>::value,

Everything works with Opaque if I take &Task instead of Box<Task>, which would be fine except I need to return an (owned) Task.