config: Use _LIBCPP_TYPE_VIS_ONLY with enum class

An enum class has associated type info. In the Microsoft ABI, type info
is emitted in the COMDAT section and isn't exported, so clang rightfully
complains about __declspec(dllexport) being unused for an enum class.
On other platforms, we still want to export the type info.

Differential Revision: https://reviews.llvm.org/D24065

llvm-svn: 281264