[libcxx-commits] [libcxx] [libcxx] Define `_LIBCPP_HAS_C8RTOMB_MBRTOC8` to true if compiling with clang (PR #165439)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

I _guess_ a workaround would be adding

```C++
#    if _LIBCPP_HAS_C8RTOMB_MBRTOC8
using ::mbrtoc8 _LIBCPP_USING_IF_EXISTS;
using ::c8rtomb _LIBCPP_USING_IF_EXISTS;
#    endif
```

to the global namespace.

Also, please resolve merge conflicts first.

https://github.com/llvm/llvm-project/pull/165439