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.
clang-format suggested style edits found: