[clang] [llvm] [DirectX][DXIL] Set DXIL Version in DXIL target triple based on shader model version (PR #90809)

[Justin Bogner via cfe-commits]

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

Other than [my comment here](https://github.com/llvm/llvm-project/pull/90809/files#r1589771652) about what to do in getArchName this looks reasonable.

The string handling in `normalize` feels a bit unwieldy, but I don't have any obvious thoughts for how to simplify it.

https://github.com/llvm/llvm-project/pull/90809