rustc should fail CI on allow(incomplete_features)

[BoxyUwU]

I do not think it is a good idea for rustc to be depending on incomplete features, it doesn't feel particularly fair to people working on said features that when they attempt to make progress on a broken feature they would have to deal with cfg(bootstrap) pain and generally fixing rustc itself from usage of the feature. Also just generally depending on broken or unsound features in rustc is probably not good 😆

Recently stumbled on #107937 which attempts to enable generic_const_expr a (very) broken/incomplete feature.

As far as I can tell there's only one usage of allow(incomplete_features) and that's to enable adt_const_params which will likely be moved off of incomplete_features soonish