Clean up StorageClass business

[mstange]

No description provided.

[jrmuizel]

Can you remind me what the problem is here?

[mstange]

write_pre writes out a StorageClass for some types and does return Ok(()); for others. We need to find out whether those early returns are ok, or if the storage class needs to be handled on them as well. If it does, we can probably simplify some things: If all types need a storage class, maybe Type should be a pair of StorageClass and the actual type enum.