Add `never_patterns` feature gate This PR adds the feature gate and most basic parsing for the experimental `never_patterns` feature. See the tracking issue (https://github.com/rust-lang/rust/issues/118155) for details on the experiment. `@scottmcm` has agreed to be my lang-team liaison for this experiment.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.