[clang] [llvm] [HLSL][RootSignature] Add `hlsl-rootsig-ver` option to specify root signature version (PR #144813)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

The implementation all looks fairly straightforward here, but shouldn't we be naming the `clang-dxc` flag to match `dxc`'s flag that does this? That is, `-force-rootsig-ver`.

https://github.com/llvm/llvm-project/pull/144813