[clang] [llvm] Reland "[HLSL] Implement the reflect HLSL function" (PR #125599)

[Justin Bogner via cfe-commits]

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

I have some misgivings about the way we're partially introducing the error handling here, but my disagreements are largely philosophical and procedural. Since we've worked around the issues with the hwasan bot at this point it's probably fine to get this PR in and leave the improvements to error handling to #124045

https://github.com/llvm/llvm-project/pull/125599