ByteArray# as a foreign import argument?

[Shao, Cheng]

Hello devs,

I've been trying to figure out how to pass lifted types as foreign
types, then encountered the following code in the `DsCCall` module
(https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/deSugar/DsCCall.hs#L172):

```
  -- Byte-arrays, both mutable and otherwise; hack warning
  -- We're looking for values of type ByteArray, MutableByteArray
  --    data ByteArray          ix = ByteArray        ix ix ByteArray#
  --    data MutableByteArray s ix = MutableByteArray ix ix
(MutableByteArray# s)
  | is_product_type &&
    data_con_arity == 3 &&
    isJust maybe_arg3_tycon &&
    (arg3_tycon ==  byteArrayPrimTyCon ||
     arg3_tycon ==  mutableByteArrayPrimTyCon)
  = do case_bndr <- newSysLocalDs arg_ty
       vars@[_l_var, _r_var, arr_cts_var] <- newSysLocalsDs data_con_arg_tys
       return (Var arr_cts_var,
               \ body -> Case arg case_bndr (exprType body) [(DataAlt
data_con,vars,body)]
              )
```

It seems we allow a "ByteArray" type as a foreign import argument, if
the third field of the datacon is a ByteArray# or MutableByteArray#.
But I can't find such a ByteArray type definition in today's common
packages. What's the rationale for this piece of code?

Cheers,
Cheng