Flambda2 layouts: Fix partial application wrapper result arity

[ccasin]

This fixes an issue where an incorrect result arity on a partial application wrapper will trip a couple asserts in flambda2 (and would generate incorrect code without the asserts). The basic issue is two places where the result arity of a partial application (which is always value) is incorrectly used as the result arity of the fully applied version after wrapping (which might not be value).

I don't think this error can be observed on the current main branch because you need alternate layouts. But I can see it in #1528 on programs like this:

external of_float : (float[@local_opt]) -> float# = "%unbox_float"

let[@inline never] myapp f (x : float#) = f x

let id_float = myapp (fun x -> x)

let make_pi =
  let x = id_float (of_float 3.14) in ()

Both places are in Closure_conversion.wrap_partial_application - the rest of this is just plumbing to get the correct result arity to that function.

I haven't added a test here because I'm not sure it's possible without another layout, but I have tested applying this diff to #1528, which gets the tests to pass on flambda2 on my machine. I've just pushed it there and we'll see if the CI validates that.

[goldfirere]

Merging with @Ekdohibs 's approval and in @mshinwell's absence.