Print captures precisely in borrow checker

[arora-aman]

The borrow checker just prints the root variable when printing captures from within the context of the closure, we should update this to be print the captured path precisely.

Ideally, we do this once rust-lang/rust#80635 lands because it introduces a function to prints paths precisely.

[arora-aman]

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/fn.place_to_string_for_capture.html

[arora-aman]

Search for FIXME(project-rfc_2229#36); https://oli-obk.github.io/fixmeh/

[arora-aman]

error[E0502]: cannot borrow `p` as mutable because it is also borrowed as immutable
  --> $DIR/borrowck-2.rs:12:17
   |
LL |     let y = &p.y;
   |             ---- immutable borrow occurs here
LL |     let mut c = || {
   |                 ^^ mutable borrow occurs here
LL |        println!("{:?}", p);
   |                         - `p` is captured because of use here
LL |        let x = &mut p.x;
   |                     --- `p` is borrowed mutably because of use  here
...
LL |     println!("{}", y);
   |                    - immutable borrow later used here