[clang] Cir cuda builtin vars (PR #195539)

[Akimasa Watanuki via cfe-commits]

https://github.com/Men-cotton requested changes to this pull request.

Thanks for working on this. Some changes seem to go beyond the behavior covered by this PR, so please take another look and either keep them scoped out or add the corresponding tests.

https://github.com/llvm/llvm-project/pull/195539