UniquePtr of pure virtual opaque extern C type doesn't compile

[dtolnay]

Repro:

// lib.rs

#[cxx::bridge]
mod ffi {
    extern "C" {
        include!("repro.h");

        type Virtual;
        fn make() -> UniquePtr<Virtual>;
    }
}

// repro.h

#pragma once

struct Virtual {
  virtual void function() = 0;
};

std::unique_ptr<Virtual> make();

This fails to compile with:

repro.rs.cc: In function ‘void cxxbridge02$unique_ptr$Virtual$new(std::unique_ptr<Virtual>*, Virtual*)’:
repro.rs.cc:599:85: error: invalid new-expression of abstract class type ‘Virtual’
  |   new (ptr) ::std::unique_ptr<Virtual>(new Virtual(::std::move(*value)));
  |                                                                       ^
repro.h:11:8: note:   because the following virtual functions are pure within ‘Virtual’:
  | struct Virtual {
  |        ^~~~~~~
repro.h:12:16: note: 	‘virtual void Virtual::function()’
  |   virtual void function() = 0;
  |                ^~~~~~~~

The error is inside of the shim we emit for UniquePtr::new:

void cxxbridge02$unique_ptr$Virtual$new(::std::unique_ptr<Virtual> *ptr, Virtual *value) noexcept {
  new (ptr) ::std::unique_ptr<Virtual>(new Virtual(::std::move(*value)));
}

But we should just not emit that shim for opaque C types. The signature of UniquePtr::new is fn(T) -> UniquePtr<T> which is uncallable when T is an opaque C type because those can never exist by value on the Rust side of the bridge.