fix -Winjected-class-name warnings in Clang

When compiling with Clang 5.0.1 one gets two warnings about nested type
specifiers that were probably not intended to be there in the first
place. The first one is asking a *_t type trait for ::type and the
second one is asking for Entity::Entity.

The exact warnings given by Clang were:

/duneci/modules/dune-subgrid/dune/subgrid/common/variant.hh:31:66: warning: ISO C++ specifies that qualified reference to 'type' is a constructor name rather than a type in this context, despite preceding 'typename' keyword [-Winjected-class-name]
    using Storage = typename std::aligned_storage_t<sizeof(Tp)>::type;
                                                                 ^

/duneci/modules/dune-subgrid/dune/subgrid/subgrid.hh:1190:67: warning: ISO C++ specifies that qualified reference to 'Entity' is a constructor name rather than a type in this context, despite preceding 'typename' keyword [-Winjected-class-name]
            typedef typename HostGrid::template Codim<0>::Entity::Entity HostGridElement;
                                                                  ^