Improve logging of capture analysis

[roxelo]

This PR aims to improve the logging of capture analysis by reducing the verbosity of the logs.

For instance, consider the following test case: src/test/ui/closures/2229_closure_analysis/simple-struct-min-capture.rs

Previously, the logs would have outputted the following to stdout:

For closure=DefId(0:4 ~ simple_struct_min_capture[317d]::main::{closure#0}): Using new-style capture analysis
For closure=DefId(0:4 ~ simple_struct_min_capture[317d]::main::{closure#0}): capture information: {
    Place {
        base_ty: Point,
        base: Upvar(
            UpvarId(HirId { owner: DefId(0:3 ~ simple_struct_min_capture[317d]::main), local_id: 1 };`p`;DefId(0:4 ~ simple_struct_min_capture[317d]::main::{closure#0})),
        ),
        projections: [
            Projection {
                ty: i32,
                kind: Field(
                    0,
                    0,
                ),
            },
        ],
    }: CaptureInfo {
        expr_id: Some(
            HirId {
                owner: DefId(0:3 ~ simple_struct_min_capture[317d]::main),
                local_id: 15,
            },
        ),
        capture_kind: ByRef(
            UpvarBorrow(MutBorrow, '_#34r),
        ),
    },
    Place {
        base_ty: Point,
        base: Upvar(
            UpvarId(HirId { owner: DefId(0:3 ~ simple_struct_min_capture[317d]::main), local_id: 1 };`p`;DefId(0:4 ~ simple_struct_min_capture[317d]::main::{closure#0})),
        ),
        projections: [],
    }: CaptureInfo {
        expr_id: Some(
            HirId {
                owner: DefId(0:3 ~ simple_struct_min_capture[317d]::main),
                local_id: 35,
            },
        ),
        capture_kind: ByRef(
            UpvarBorrow(ImmBorrow, '_#35r),
        ),
    },
}

and the following to stderr:

error[E0658]: attributes on expressions are experimental
  --> $DIR/simple-struct-min-capture.rs:25:17
   |
LL |     let mut c = #[rustc_capture_analysis]
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable

warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/simple-struct-min-capture.rs:3:12
   |
LL | #![feature(capture_disjoint_fields)]
   |            ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default
   = note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0658`.

After the changes, the following is being outputted to stderr:

error[E0658]: attributes on expressions are experimental
  --> src/test/ui/closures/2229_closure_analysis/simple-struct-min-capture.rs:25:17
   |
25 |     let mut c = #[rustc_capture_analysis]
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable

warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/test/ui/closures/2229_closure_analysis/simple-struct-min-capture.rs:3:12
  |
3 | #![feature(capture_disjoint_fields)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information

For closure=DefId(0:4 ~ simple_struct_min_capture[317d]::main::{closure#0}): Using new-style capture analysis
error: Capturing p[(0, 0)] -> MutBorrow
  --> src/test/ui/closures/2229_closure_analysis/simple-struct-min-capture.rs:28:9
   |
28 |         p.x += 10;
   |         ^^^

error: Capturing p[] -> ImmBorrow
  --> src/test/ui/closures/2229_closure_analysis/simple-struct-min-capture.rs:29:26
   |
29 |         println!("{:?}", p);
   |                          ^

error: aborting due to 3 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0658`.

Closes rust-lang/project-rfc-2229#19

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