improve case with one named, one anonymous lifetime parameter

[nikomatsakis]

@gaurikholkar is working on a patch to address the case of conflict between one named and one anonymous lifetime parameter. Driving example (with the rough error message that we want displayed inline):

fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
                       - consider changing the type of `y` to `&'a i32`
    if x > y { x } else { y }
                          ^ this reference must have lifetime 'a
}

[gaurikholkar-zz]

#43434

[gaurikholkar-zz]

#43433

[gaurikholkar-zz]

@nikomatsakis, we aren't handling Impls here

[estebank]

This case seems to be correctly handled now, and the other linked issues cover other improvements over this.