Add AllowInvalidPlacement check to IsCellBuildable.

Fixes #5902.
* You are right. `a` is usually `Actor` so `cell` may be better or `c` if you want to follow the obsession of the previous developer abbreviating everything beyond unreadable. I also dislike one letter shortcuts, because it is hard to hover over them to see the type via the tooltip. However, this is also meant to be a quick fix that does not touch too much code.