[clang] [llvm] Reland "[HLSL][RootSignature] Implement serialization of RootConstants and RootFlags" (PR #143019)

[Finn Plummer via cfe-commits]

inbelic wrote:

Based on feedback here: https://github.com/llvm/llvm-project/pull/143198. I will address this feedback before this can be merged.

https://github.com/llvm/llvm-project/pull/143019