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

[Damyan Pepper via llvm-commits]

https://github.com/damyanp approved this pull request.

I think this change looks ok to me, but I'd love to have a better understanding of the plan for how these DXIL versions / sub archs / SM versions all work together in clang/clang-dxc/llvm.

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