Allow calling pure virtual functions

[adetaylor]

At the moment, trying to generate code for a pure virtual class says:

gen/content/browser/gen0.cc:134:5: error: field type 'content::RenderFrameHost' is an abstract class
  T value;
    ^
gen/content/browser/gen0.cc:344:91: note: in instantiation of template class 'rust::MaybeUninit<content::RenderFrameHost>' requested here
  ::content::RenderFrameHost *uninit = reinterpret_cast<::content::RenderFrameHost *>(new ::rust::MaybeUninit<::content::RenderFrameHost>);
                                                                                          ^
../../ipc/ipc_listener.h:25:16: note: unimplemented pure virtual method 'OnMessageReceived' in 'RenderFrameHost'
  virtual bool OnMessageReceived(const Message& message) = 0;
               ^

Relevant bits of code from the generated C++:

template <typename T>
union MaybeUninit {
  T value;
  void *operator new(::std::size_t sz) { return detail::operator_new<T>{}(sz); }
  MaybeUninit() {}
  ~MaybeUninit() {}
};

...

::content::RenderFrameHost *cxxbridge1$unique_ptr$content$RenderFrameHost$uninit(::std::unique_ptr<::content::RenderFrameHost> *ptr) noexcept {
  ::content::RenderFrameHost *uninit = reinterpret_cast<::content::RenderFrameHost *>(new ::rust::MaybeUninit<::content::RenderFrameHost>);
  ::new (ptr) ::std::unique_ptr<::content::RenderFrameHost>(uninit);
  return uninit;
}

This is, I think, because we are doing impl UniquePtr<content::RenderFrameHost> in our generated cxx::bridge.

[adetaylor]

Sitrep:

#include <cstdint>
class A {
public:
    virtual uint32_t get_val() const = 0;
};
class B : public A {
public:
    virtual uint32_t get_val() const { return 3; }
};
const B b;
inline const A* get_a() { return &b; };

Bindgen generates:

pub mod root {
        #[allow(unused_imports)]
        use self::super::root;
        #[repr(C)]
        pub struct A__bindgen_vtable(::std::os::raw::c_void);
        #[repr(C)]
        pub struct A {
            pub vtable_: *const A__bindgen_vtable,
        }
        extern "C" {
            #[link_name = "\u{1}__Z5get_av"]
            pub fn get_a() -> *const root::A;
        }
    }
     }

and thus we don't find out about virtual functions at all. rust-lang/rust-bindgen#1143

Options:

• Fix upstream bindgen issue (but this isn't quite ideal for us; we don't want to call functions using the vtable from Rust; we simply want to know of their existence so we can generate C++ wrapper functions)
• Modify our autocxx-bindgen 'temporary' fork to output some metadata about these
• Eventually switch away from bindgen and use libclang directly