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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@213116 91177308-0d34-0410-b5e6-96231b3b80d8