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

[Justin Lebar via cfe-commits]

jlebar wrote:

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

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