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

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

What's the plan for when a subarch isn't specified (ie, `dxil-pc-shadermodel-6.0`)? Will we need a mapping somewhere else that applies a default dxil version for a given shader model? Will we just have to check both the dxil subarch and the shader model anywhere where it matters?

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