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

[Justin Lebar via cfe-commits]

jlebar 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.

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