Gruesome fix in CorePrep to fix embarassing Trac #4121
This is a long-lurking bug that has been flushed into the open by other arity-related changes. There's a long comment Note [CafInfo and floating] to explain. I really hate the contortions we have to do through to keep correct CafRef information on top-level binders. The Right Thing, I believe, is to compute CAF and arity information later, and merge it into the interface-file information when the latter is generated. But for now, this hackily fixes the problem.