[libc][stdlib] implement wcstombs

[nickdesaulniers]

In order to test various functions from wchar.h, we need to be able to print the actual vs expected results. To do so, we must be able to convert wide character strings back to c strings. The canonical way to do so in C is via wcstombs. wcstombs can be implemented in terms of wcsrtombs.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

In order to test various functions from wchar.h, we need to be able to print the actual vs expected results. To do so, we must be able to convert wide character strings back to c strings. The canonical way to do so in C is via `wcstombs`. `wcstombs` can be implemented in terms of `wcsrtombs`.

[michaelrj-google]

Completed in #149421