[libcxx] Make the visibility attributes consistent for __narrow_to_utf8/__widen_from_utf8

Use the same visiblity attributes as for all other template
specializations in the same file; declare the specialization itself
using _LIBCPP_TYPE_VIS, and don't use _LIBCPP_EXPORTED_FROM_ABI on
the destructor. Methods that are excluded from the ABI are marked
with _LIBCPP_INLINE_VISIBILITY.

This makes the vtable exported from DLL builds of libc++. Practically,
it doesn't make any difference for the CI configuration, but it
can make a difference in mingw setups.

Differential Revision: https://reviews.llvm.org/D102717