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

[Farzon Lotfi via llvm-commits]

farzonl wrote:

> 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?

The graceful exit should be sufficient. We don't expose the clang builtin when targeting spirv32 or spirv64 and for now we are the only consumers of this intrinsic. Like I said in the last comment to only way to trigger this crash is with hand spun llvmir. To me making this not crash if you aren't in a supported environment is good enough. 

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