[clang] [HLSL] Implement the `lit` intrinsic (PR #134171)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

I'm concerned that you had to remove your change in `clang/lib/Headers/hlsl.h`. I think you did it because you needed a template definition before you could specialize it. My concern is that now the generic template is gated by `#if __HLSL_VERSION <= __HLSL_202x` which mean you should see an error at shader compile time if the HLSL version were 202y.

https://github.com/llvm/llvm-project/pull/134171