Support calling reference qualified member function from Rust

[dtolnay]

For example, this method from folly::dynamic: https://github.com/facebook/folly/blob/f604b03c5e0305860326f77c4562cba27547db38/folly/dynamic.h#L311

Simplified, the data structure looks like:

struct dynamic {
  double getDouble() const &;
};

If we currently try binding this from CXX as follows, then it fails to compile.

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        type dynamic;
        fn getDouble(self: &dynamic) -> f64;
    }
}

The generated code contains something along the lines of:

double (dynamic::*getDouble$)() const = &dynamic::getDouble;

main.cc:6:21: error: cannot initialize a variable of type 'double (dynamic::*)() const' with an rvalue of type 'double (dynamic::*)() const &'
  double (dynamic::*getDouble$)() const = &dynamic::getDouble;
                    ^                     ~~~~~~~~~~~~~~~~~~~

[dtolnay]

Maybe like this:

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        type dynamic;

        #[ref_qualifier = "&"]  // or "&&"
        fn getDouble(&self) -> f64;
    }
}

The attribute uses the terminology from https://en.cppreference.com/w/cpp/language/member_functions#ref-qualified_member_functions.