swift234

[Serialization] Stop using DeclID et al for "the next available ID".

This is effectively fallout from 36a44cf3 where we switched representations of DeclID
and friends, but when I went to add ++ and -- to llvm::PointerEmbeddedInt I realized
it wasn't really intended to be mutated in place, i.e. it doesn't make sense to
increment a DeclID. Represent counters as plain integers instead that get converted
to DeclID when they are first used.