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

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

> @RKSimon The builtin currently can't be used with `constexpr`. Support for constant evaluation is planned for a follow-up PR unless you would like me to add it in this one. Should I remove the `Constexpr` attribute from the builtin in Builtins.td for now?

Yes please!

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