[clang] [clang] Implement __builtin_popcountg (PR #82359)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

Perhaps worth rebasing then force pushing to this branch to rekick the build bots?

Thanks for implementing this @overmighty !  There's a few more builtins like this that would simplify implementations of more C23 library additions. Let me know if you're interested in those, too.

https://github.com/llvm/llvm-project/pull/82359