[clang] [llvm] [HLSL][SPIRV][DXIL] Implement `WaveActiveSum` intrinsic (PR #118580)

[Finn Plummer via llvm-commits]

https://github.com/inbelic commented:

We also decided to use a different naming convention (`reduce` instead of `active`) for the intrinsics since my pr, following [here](https://github.com/llvm/wg-hlsl/pull/99)



https://github.com/llvm/llvm-project/pull/118580