[clang] [llvm] [NFC][HLSL][RootSignature] Split up `HLSLRootSignatureUtils` (PR #146124)

[Alex Sepkowski via llvm-commits]

https://github.com/alsepkow commented:

LGTM outside of a few minor comments you may choose to address or not.

https://github.com/llvm/llvm-project/pull/146124