Positive coverage: prove_ty_is_ref / ref

1 test exercised this rule:

Source location: tests/borrowck.rs:2277

fn min_problem_case_4<'a>(list: &mut 'a Map, list2: &mut 'a Map) -> u32 {
    exists<'r0> {
        let num: &mut 'r0 u32 = &mut 'r0 (*list).value;
        list = &mut 'a *list2;
        num;
        return 0 _ u32;
    }
}