[clang] [llvm] [NVPTX] Add 'activemask' builtin and intrinsic support (PR #79768)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Unlike the other PRs, this one has a CUDA function, `__activemask()`. Presumably we should make that one work by hacking our headers?

That is currently defined here https://github.com/llvm/llvm-project/blob/main/clang/lib/Headers/__clang_cuda_intrinsics.h#L214. I was planning on updating this to use the new instrinsic for the newer version. Alternatively we could make `__activemask` the builtin which expands to both versions, but I'm somewhat averse since we should target the instruction directly I feel.

https://github.com/llvm/llvm-project/pull/79768