[clang] [llvm] [HLSL][RootSignature] Add infastructure to parse parameters (PR #133800)

[Finn Plummer via cfe-commits]

inbelic wrote:

I don't have any strong arguments against structuring it like described and have updated accordingly.

Since we are changing the calling convention of the `parse.*`  methods to use `std::optional` I had originally also updated the `parseDescriptorTable` and `parseDescriptorTableClause` as well. But it made the diff difficult to read.

Changes to make the calling convention consistent with those methods are in the branch [here](https://github.com/inbelic/llvm-project/tree/inbelic/rs-update-call-convs). I will create the clean up pr once this one merges

https://github.com/llvm/llvm-project/pull/133800