Avoids many pitfalls connected with premature/early return in case
there are errors with Decl, etc. This is effectively bringing back
the old design however in a much nicer packaging, where every
mechanism related to tracking Decl's debug info is now nicely
wrapped in a single struct (aka the `DeclState`). This includes
relocation table, type arena, etc. It is now the caller's
responsibility to deinit the state (so that no memory is leaked)
after `Decl` has been analysed (or errored out). The caller here
is typically a linker such as `Elf` or `MachO`.