De-abuse TyKind::Error in exhaustiveness checking
Replaces https://github.com/rust-lang/rust/pull/71074. Context: https://github.com/rust-lang/rust/issues/70866.
In order to remove the use of `TyKind::Error`, I had to make sure we skip over those fields whose inhabitedness should not be observed. This is potentially error-prone however, since we must be careful not to mix filtered and unfiltered lists of patterns. I managed to hide away most of the filtering behind a new `Fields` struct, that I used everywhere relevant. I quite like the result; I think the twin concepts of `Constructor` and `Fields` make a good mental model.
As usual, I tried to separate commits that shuffle code around from commits that require more thought to review.
cc @varkor @Centril
9310e3bd4f