Suggest fix for `borrowed_box` return value

[camsteffen]

It would be nice if borrowed_box suggested a fix for the returned value in addition to the function signature.

Example from #6723:

pub trait Inner{}
pub struct Wrapper(Box<dyn Inner>);

pub unsafe fn get_inner(wrapper: *mut Wrapper) -> &'static Box<dyn Inner> {
    &(*wrapper).0
}