[libcxx] Add missing _LIBCPP_FUNC_VIS on a few win32 locale functions

These functions are called directly from the public installed
headers, and thus need to be exported in DLL builds, just like
some other functions in the same header (e.g. snprintf_l).

This fixes e.g. test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp
in mingw configurations.

Differential Revision: https://reviews.llvm.org/D91328