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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > 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.
> 
> Yes, I agree that the builtin shouldn't have a "polyfill". At least, the LLVM builtin should not have a polyfill -- I guess I'm neutral on whether the clang builtin does.
> 
> You can change clang in this same patch if you want, but if you want to do it separately, that's also fine by me. I'll approve this one. I think that covers all my outstanding review requests from you? LMK if I missed any.

Thanks for the reviews. I'll probably have one for `nanosleep` coming in soonish, but for now I believe you've got it covered.

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