- Introduced with the C++20 <bit> header
- The problem with __builtin_c(l|t)z() is that 0 as input has an
undefined result (*) and the code did not always check. The std::
versions do not have this issue.
- In some cases, we continue to use buildin_c(l|t)z(), (e.g. in
src/Common/BitHelpers.h) because the std:: versions only accept
unsigned inputs (and they also check that) and the casting would be
ugly.
(*) https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
