[clang] [llvm] [NFC][HLSL][DirectX] Let `HLSLRootSignature` reuse the `dxbc` defined enums (PR #145986)

[Justin Bogner via llvm-commits]

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

looks good, one more comment about using enum values rather than bit patterns in a test

https://github.com/llvm/llvm-project/pull/145986