Change Windows decoration on some base classes

Mark the base classes for time_get_byname and time_get as _LIBCPP_TYPE_VIS_ONLY
rather than _LIBCPP_TYPE_VIS.  These base classes are templated types and cannot
be stored with export dll storage.

Fixes compilation with _LIBCPP_DLL for Windows when the time_get and
time_get_byname classes are used.