Allow marshalling of `(# #)` in FFI calls.

Summary

Sometimes there is no need for a ffi function which takes no arguments to require ordering via state.

To me the natural type to represent this is something like: (# #) -> Int#

However currently this isn't legal.

Steps to reproduce

foreign import prim "somePrimFun" someStuff3 :: (# #) -> Int#

Expected behavior

The above import should work.

Environment

• GHC version used:

Optional:

• Operating System:
• System Architecture: