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

[Justin Bogner via llvm-commits]

bogner wrote:

@jurahul Yeah that looks like the right fix - looks like we changed a header-only dependency to a real dependency without noticing we didn't have the dependency in the CMakeLists already.

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