patterns: reject raw pointers that are not just integers
Matching against `0 as *const i32` is fine, matching against `&42 as *const i32` is not.
This extends the existing check against function pointers and wide pointers: we now uniformly reject all these pointer types during valtree construction, and then later lint because of that. See [here](https://github.com/rust-lang/rust/pull/116930#issuecomment-1784654073) for some more explanation and context.
Also fixes https://github.com/rust-lang/rust/issues/116929.
Cc `@oli-obk` `@lcnr`
exhaustiveness: Rework constructor splitting
`SplitWildcard` was pretty opaque. I replaced it with a more legible abstraction: `ConstructorSet` represents the set of constructors for patterns of a given type. This clarifies responsibilities: `ConstructorSet` handles one clear task, and diagnostic-related shenanigans can be done separately.
I'm quite excited, I had has this in mind for years but could never quite introduce it. This opens up possibilities, including type-specific optimisations (like using a `FxHashSet` to collect enum variants, which had been [hackily attempted some years ago](https://github.com/rust-lang/rust/pull/76918)), my one-pass rewrite (https://github.com/rust-lang/rust/pull/116042), and future librarification.
