Rollup merge of #120517 - Nadrieril:lower-never-as-wildcard, r=compiler-errors

never patterns: It is correct to lower `!` to `_`.

This is just a comment update but a non-trivial one: it is correct to lower `!` patterns as `_`. The reasoning is that `!` matches all the possible values of the type, since the type is empty. Moreover, we do want to warn that the `Err` is redundant in:
```rust
match x {
  !,
  Err(!),
}
```
which is consistent with `!` behaving like a wildcard.

I did try to introduce `Constructor::Never` and it ended up needing to behave exactly like `Constructor::Wildcard`.

r? ```@compiler-errors```

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