fix: Fix normalization in the new solver #20647
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More correctly, a `TyKind::AssociatedType` is not the same as `TyKind::Projection`. We used to map next-solver `TyKind::Alias` to Chalk's `TyKind::AssociatedType`. This is very incorrect, as `AssociatedType` is assumed to be fully normalized, and caused a lot of type mismatches. Unfortunately fixing this causes a lot of stack overflows, because the next solver doesn't have something akin to `AssociatedType` so we normalize again and again. The reason is that is the lazy-normalization nature of the next solver, which means we need to stop normalizing everything. This will be fixed in the next commit.
Previously normalization was broken, which caused a lot of fake errors. This fix most type mismatches of the new solver, and it also reverts many test regressions. The downside is that now `chalk_ir::TyKind::AssociatedType`/`chalk_ir::TyKind::Alias` cannot be trusted anymore with their roles, namely: `AssociatedType` is always fully normalized and `Alias` only if it can possibly be normalized further. That seems okay as the new solver does not have this distinction at all (due to it being a lazy normalizer), so this will only hold for the migration time. This does mean we have to change some APIs, notably `hir::Type::walk()` and `TyFingerprint`, to treat `Alias` the same as `AssociatedType`. Another small thing this commit does is to isolate processing of user-written types (currently involving replacing error types and normalizing, but in the future validation will also be needed) to separate functions.
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This fixes a lot of test regressions (@ShoyuVanilla including ones from your PR) and type mismatches.
Fixes #20555.