[llvm] [DXIL] Add DXIL SubArch to correspond to version number (PR #89125)

[Chris B via llvm-commits]

https://github.com/llvm-beanz approved this pull request.

I'm happy with this change as it is with one request.

@bharadwajy, if you're not going to do the clang change with this patch, can you file an issue to track that because we will need that change to land before we can start making the backend depend on the sub-architecture (otherwise we're always going to set it wrong).

https://github.com/llvm/llvm-project/pull/89125