[libc][wchar] implement wcrtomb

[nickdesaulniers]

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

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

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

[nickdesaulniers]

I think in order to support windows AND unixes, we'll want to implement wcrtomb in terms of EITHER c16rtomb for windows targets OR c32rtomb for non-windows targets.

[michaelrj-google]

Completed in #145554