MIR borrowck: closure borrow errors mention the closure instead of the upvar

[arielb1]

e.g. the test for E0504:

struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let x = move || {
        println!("child function: {}", fancy_num.num); //~ ERROR E0504
    };

    x();
    println!("main function: {}", fancy_ref.num);
}

Gives the following errors with -Z borrowck=compare:

error[E0504]: cannot move `fancy_num` into closure because it is borrowed (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0504.rs:20:40
   |
17 |     let fancy_ref = &fancy_num;
   |                      --------- borrow of `fancy_num` occurs here
...
20 |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
   |                                        ^^^^^^^^^ move into closure occurs here

error[E0505]: cannot move out of `fancy_num` because it is borrowed (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0504.rs:19:13
   |
17 |       let fancy_ref = &fancy_num;
   |                       ---------- borrow of `fancy_num` occurs here
18 | 
19 |       let x = move || {
   |  _____________^
20 | |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
21 | |     };
   | |_____^ move out of `fancy_num` occurs here

error: aborting due to 2 previous errors

If the closure is big, it can be pretty annoying to find the offending upvar reference, so the AST error message is better.

[arielb1]

Dupe of #46599.