[libc++][nfc] remove duplicated __to_unsigned.

[libc++][nfc] remove duplicated __to_unsigned.

Both `<type_traits>` and `<charconv>` implemented this function with
different names and a slightly different behavior. This removes the
version in `<charconv>` and improves the version in `<typetraits>`.

- The code can be used again in C++11.
-  The original claimed C++14 support, but `[[nodiscard]]` is not
   available in  C++14.
- Adds `_LIBCPP_INLINE_VISIBILITY`.

Reviewed By: zoecarver, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D102332