[libc++] Optimize std::has_single_bit (#133063)
Clang translates most implementations of has_single_bit to `(v ^ (v-1))
> v-1` - except the one definition libc++ actually uses.
Proof of correctness: https://godbolt.org/z/d61bxW4r1
(Could also be fixed by teaching Clang to optimize better, but making
source match output feels clearer to me. And it improves unoptimized
performance.)
