[llvm] [DirectX] Refactor RootSignature Backend to remove `to_underlying` from Root Parameter Header (PR #154249)

[via llvm-commits]

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joaosaffran wrote:

It not from `Object/DXContainer.h`, it is from some values defined inside `BinaryFormat/DXContainer.h` mainly the `RTS0` namespace

https://github.com/llvm/llvm-project/pull/154249