Unpack warning when building GHC

@alanz reports on ghc-devs:

I have seen the following warning on master for some time

compiler/GHC/Core/TyCon.hs:1540:5: warning:
   • Ignoring unusable UNPACK pragma
       on the first argument of ‘BoxedRep’
   • In the definition of data constructor ‘BoxedRep’
     In the data type declaration for ‘PrimRep’
    |
1540 |   | BoxedRep {-# UNPACK #-} !(Maybe Levity) -- ^ Boxed, heap value
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Is it something that needs to be fixed? Can the code be updated to remove the warning?

I'll fix it