Set _LIBCPP_DLL_VIS on _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS in MinGW mode

Contrary to MSVC, MinGW compilers wants the dllexport attribute on
the declaration of an explicit template instantiation, not on the
definition.

Differential Revision: https://reviews.llvm.org/D61123

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359227 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed