Rollup merge of #53395 - varkor:__Nonexhaustive-to-non_exhaustive, r=shepmaster

Use #[non_exhaustive] on internal enums

This replaces `__Nonexhaustive` variants (and variants thereof)  with `#[non_exhaustive]`. These were mostly unstable previously, with the exception of the `cloudabi` enums and `Level` in proc_macro: these were `#[doc(hidden)]`, so clearly intended not to be used directly. It should be safe to replace all of these.