Rollup merge of #153785 - amandasystems:debug-type-test, r=wesleywiser

Hand-written Debug implementation for `TypeTest`

This adds a hand-written debug format for `TypeTest`s that at least was helpful for me when debugging because I always struggle to remember which component is which. It formats a type test using the Unicode turnstile symbol (for "computes") to illustrate that the test encodes a typing rule that, if it holds, produces a conclusion.

The format is: `TypeTest from {originating span} {bound}  ⊢ T: 'lower_bound`, where `T` is the generic type being tested and `lower_bound` is the lower bound. Bounds are formatted as you would expect, but where the region for `'lower_bound` is included in the outlives constraints for context. I resisted the urge to turn `ALL [A, ..., Z]` into `[A  ∧ ...  ∧ Z]` etc.

## What it looks like

Here's an example of a simple type test from the test suite that says that some type `I/#0` will be lower-bounded (outlive) `?4` iff `'?1: '?4`:

Before:
```
$ RUSTC_LOG="rustc_borrowck::region_infer=debug" rustc +stage1 tests/ui/associated-types/associated-types-eq-3.rs
...
DEBUG rustc_borrowck::region_infer type tests: [
     TypeTest {
         generic_kind: I/#0,
         lower_bound: '?4,
         span: tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0),
         verify_bound: OutlivedBy(
             '?1,
         ),
     },
     TypeTest {
         generic_kind: I/#0,
         lower_bound: '?4,
         span: tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0),
         verify_bound: OutlivedBy(
             '?1,
         ),
     },
 ]
...
```

After:
```
$ RUSTC_LOG="rustc_borrowck::region_infer=debug" rustc +stage1 tests/ui/associated-types/associated-types-eq-3.rs
...
 DEBUG rustc_borrowck::region_infer type tests: [
     TypeTest from tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0)['?1: '?4] ⊢ I/#0: '?4,
     TypeTest from tests/ui/associated-types/associated-types-eq-3.rs:21:18: 21:25 (#0)['?1: '?4] ⊢ I/#0: '?4,
 ]
...
```