[llvm] [HLSL][RootSignature] Implement serialization of `RootConstants` and `RootFlags` (PR #141130)

[Finn Plummer via llvm-commits]

inbelic wrote:

@jplehr Thanks for letting me know. This commit is responsible. I will revert and reland with the required linking change to:
`clang/unittests/Parse/CMakeLists.txt`

https://github.com/llvm/llvm-project/pull/141130