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

[S. Bharadwaj Yadavalli via llvm-commits]

bharadwajy wrote:

> 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?

I propose defaulting to version 1.0 when SubArch is not explicitly specified. 

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