Allow mutable return reference on signatures with any lifetime in argument

[dtolnay]

Fixes #765.

[jaques-sam]

Hello, I don't really get why an argument needs to be provided in that case.
How do I define an extern C++ function call like this one?:

Logger &logger();

I tried this, but I still get this weird error:

     ┌─ src/main.rs:32:9
     │
  32 │         fn logger<'static>() -> Pin<&'static mut Logger>;  // not working :'(
     │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ &mut return type is not allowed unless there is a &mut argument