[libc] [llvm] [libc][c11] Add stdio.h's rename() function (PR #85068)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> https://pubs.opengroup.org/onlinepubs/9699919799/ seems to include a renameat function.

That can be fixed in a distinct PR.  For now, https://github.com/llvm/llvm-project/issues/84980 is just concerned with minimally supporting libc++.

https://github.com/llvm/llvm-project/pull/85068