[libc-commits] [libc] trailing ones (PR #80459)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

Note to reviewers, this commit is stacked on top of https://github.com/llvm/llvm-project/pull/80344. Only the commit "
[libc][stdbit] implement stdc_trailing_ones (C23)" (and subsequent pushes) should be reviewed.

https://github.com/llvm/llvm-project/pull/80459