[clang] [llvm] [HLSL] Implement the `reflect` HLSL function (PR #122992)

[Finn Plummer via cfe-commits]

https://github.com/inbelic approved this pull request.

This pr LGTM once the graceful exit comment is handled.

With respect to [this comment](https://github.com/llvm/llvm-project/pull/122992#discussion_r1917127000). Is the final solution to just gracefully exit or is this an intermediate step and an issue will be used to track an update using inst combine later?

Will a follow-up issue be created to move the useful `Sema` helpers into a common file?

https://github.com/llvm/llvm-project/pull/122992