Improve error message for one named, one anonymous lifetime parameters

[gaurikholkar-zz]

For the following example

fn foo<'a>(x: &'a i32, y: Vec<&i32>) {
                       - consider changing the type of `y` to `std::vec::Vec<&'a i32>`
    y.push(x);
           ^ this reference must have lifetime 'a
}

can be refined to

fn foo<'a>(x: &'a i32, y: Vec<&i32>) {
                              ---- consider changing this type to `&'a i32`
    y.push(x);
           ^ this reference must have lifetime 'a
}

Underline &i32 instead of the whole type.

cc @nikomatsakis

[gaurikholkar-zz]

working on this.

[gaurikholkar-zz]

#43851

[gaurikholkar-zz]

#43700

[estebank]

Current output:

error[[E0621]](https://doc.rust-lang.org/nightly/error_codes/E0621.html): explicit lifetime required in the type of `y`
 --> src/lib.rs:2:5
  |
1 | fn foo<'a>(x: &'a i32, mut y: Vec<&i32>) {
  |                               --------- help: add explicit lifetime `'a` to the type of `y`: `Vec<&'a i32>`
2 |     y.push(x);
  |     ^^^^^^^^^ lifetime `'a` required