feat: add non-exhaustive-let diagnostic
I want this to have a quickfix to add an else branch but I couldn't figure out how to do that, so here's the diagnostic on its own. It pretends a `let` is a match with one arm, and asks the match checking whether that match would be exhaustive.
Previously the pattern was checked based on its own type, but that was causing a panic in `match_check` (while processing e.g. `crates/hir/src/lib.rs`) so I changed it to use the initialiser's type instead, to align with the checking of actual match expressions. I think the panic can still happen, but I hear that `match_check` is going to be updated to a new version from rustc, so I'm posting this now in the hopes that the panic will magically go away when that happens.
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