Fix: non_exhaustive_omitted_patterns by filtering unstable and doc hidden variants
Fixes: #89042
Now that #86809 has been merged there are cases (std::io::ErrorKind) where unstable feature gated variants were included in warning/error messages when the feature was not turned on. This filters those variants out of the return of `SplitWildcard::new`.
Variants marked `doc(hidden)` are filtered out of the witnesses list in `Usefulness::apply_constructor`.
Probably worth a perf run 🤷 since this area can be sensitive.
d7c97a02d1