Support &self shorthand if there is only one extern type in the block

[dtolnay]

As currently implemented, Rust associated methods and C++ member functions are currently specified with self: &T syntax.

extern "C" {
    type C;
    fn f(self: &C);
}

Inside of extern blocks that contain exactly one extern type, we should allow the usual &self and &mut self shorthand as well.

extern "C" {
    type C;
    fn f(&self);
}