Function collect_nonexhaustive_missing_variants

fn collect_nonexhaustive_missing_variants<'p, 'tcx>(
    cx: &RustcPatCtxt<'p, 'tcx>,
    column: &PatternColumn<'p, RustcPatCtxt<'p, 'tcx>>,
) -> Result<Vec<WitnessPat<'p, 'tcx>>, ErrorGuaranteed>

Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned in a given column.