Generate correct adapter code for rvalue return types

[slburson]

The adapter code generated for a function returning an rvalue reference didn't compile. This shows up in even a trivial example, such as:

struct Foo {
    Foo&& mumble();
    Foo& grumble();
};

Foo& foo();

[slburson]

The diff will be a little more intelligible if you turn on "Hide whitespace".