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

[Chris B via llvm-commits]

llvm-beanz wrote:

In the clang DXC driver mode we translate target profiles to triples (i.e. `ps_6_6` to `dxil-pc-shadermodel6.6-pixel`). Clang needs two changes to utilize this:
1) translate target profile to dxil version and shader model version.
2) In the non-DXC-mode driver, we should propagate shader model version to dxil sub arch if the DXIL sub arch is not set.

I think these changes are extremely straightforward, I'm not sure why we would need a larger design discussion around it.

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