Allow using named consts in pattern types

[oli-obk]

This required a refactoring first: I had to stop using hir::Patin hir::TyKind::Pat and instead create a separate TyPat that has ConstArg for range ends instead of PatExpr. Within the type system we should be using ConstArg for all constants, as otherwise we'd be maintaining two separate const systems that could diverge. The big advantage of this PR is that we now inherit all the rules from const generics and don't have a separate system. While this makes things harder for users (const generic rules wrt what is allowed in those consts), it also means we don't accidentally allow some things like referring to assoc consts or doing math on generic consts.

[rustbot]

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

HIR ty lowering was modified

cc @fmease

[bors]

☔ The latest upstream changes (presumably #136454) made this pull request unmergeable. Please resolve the merge conflicts.