Presumably the backwards/forwards compatibility is poor simply because people don't tend to have these files sitting around, they mostly exist (at the moment) a

Presumably the backwards/forwards compatibility is poor simply because people don't tend to have these files sitting around, they mostly exist (at the moment) as part of a compilation framework and are tempfiles. If needed, it wouldn't be too for someone with a few programmers (Apple could spare a few!) to write the proper versioning to upgrade/downgrade the file format as LLVM changes?