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

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers commented:

Awesome, thanks!

Don't forget to update https://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions

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