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

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overmighty 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?

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