Support opaque C++ types that are not structs

[dtolnay]

For example we might have:

// c++

using Obj = void*;

// rust

#[cxx::bridge]
mod ffi {
    extern "C" {
        type Obj;
        fn f(obj: &Obj);
    }
}

Currently this fails to compile because we forward declare opaque C types assuming they are structs.

target/debug/build/cxxtest/out/src/main.rs.cc:5:8: error: using typedef-name ‘using Obj = void*’ after ‘struct’
    5 | struct Obj;
      |        ^~~

[notgull]

Where does the code parse opaque C types at the moment? I’d like to try to take this issue on.