Gruesome fix in CorePrep to fix embarassing Trac #4121

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.