Fix the ICE 6539
Fixes#6539
It happened because `zero_sized_map_values` used `layout_of` with types from type aliases, which is essentially the same as the ICE 4968.
---
changelog: Fix an ICE in `zero_sized_map_values`
This makes it possible to pass the `Impl` directly to functions, instead
of having to pass each of the many fields one at a time. It also
simplifies matches in many cases.
