[libc][wchar] implement wcsnrtombs

[nickdesaulniers]

In order to implement wcsrtombs, it would be helpful to reuse wcsnrtombs.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

In order to implement wcsrtombs, it would be helpful to reuse wcsnrtombs.

[michaelrj-google]

completed in #149421