Merge #8717

Merge #8717

8717: Update match checking algorithm r=iDawer a=iDawer

I've recently got interest in the match checking to extend the current algo to support reporting witnesses of non-exhaustiveness.
It appears the algo is outdated from rustc's implementation. I decided to rewrite it based on the latest rustc's version. It is a diff-based port to ra codebase. That means you can diff-compare these files to rustc.
I'm striving to keep minimal ra-related changes in the algo to make it easier to backport future changes from the upstream.

Based on upstream algorithm of version rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22)
https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

The goal of this PR is to cover the current `missing-match-arm` diagnostic.

What is remaining to do:
- [x] Error handling. The errors that are unrelated to match checking will be handled before the check. Just like how it made in rustc.
  - [x] Lowering `hir_def::expr::Pat` to `hir_ty::diagnostics::match_check::Pat`. rustc's match checking works on top of `rustc_mir_build::thir::Pat`, which is lowered from `hir::Pat` and carries some extra semantics used by the check. All unrelated checks are done there. RA could use this to rule out running the check on unimplemented cases (`Pat::ConstBlock`, etc).
  - [x] ~~Proper~~Loose typecheck of match arm patterns (https://github.com/rust-analyzer/rust-analyzer/pull/8840, https://github.com/rust-analyzer/rust-analyzer/pull/8875).
- [x] Tests from `hir_ty::diagnostics::match_check::tests`.
- [x] Clean up `todo`s
- [x] Test run on real repos https://github.com/rust-analyzer/rust-analyzer/pull/8717#issuecomment-847120265.

Co-authored-by: Dawer <7803845+iDawer@users.noreply.github.com>