This addresses points (1a) and (1b) of #19165.

- Move mkFailExpr to HsToCore/Utils, as it can be shared
- Desugar incomplete patterns and holes to an empty case,
  as in Note [Incompleteness and linearity]
- Enable linear linting of desugarer output
- Mark MultConstructor as broken. It fails Lint, but I'd like to fix this
  separately.

Metric Decrease:
    T6048