[libc] [llvm] [libc] Add more functions in CPP/bit.h (PR #73814)

[Schrodinger ZHU Yifan via llvm-commits]

SchrodingerZhu wrote:

> The implementations LGTM, but do you have plans to use all of these functions?  If they don't have a use case right now they seem like they'd be easy enough to add later.

I think popcount is used in some tests, rotl is used in hash; all previous use to clz/ctz can be changed to using CPP/bit.h instead as it is more close to cpp's std. 

https://github.com/llvm/llvm-project/pull/73814