Allow runtime construction of DeviceAdapterId

When need some way to convert user input such as an integer or
string to a device adapter id. This allows for integer to
device adapter id.

vtkm/cont/internal/DeviceAdapterTag.h

@@ -63,6 +63,8 @@ struct DeviceAdapterId
   DeviceAdapterNameType GetName() const;
 
 protected:
+  friend DeviceAdapterId make_DeviceAdapterId(vtkm::Int8 id);
+
   constexpr explicit DeviceAdapterId(vtkm::Int8 id)
     : Value(id)
   {
@@ -72,6 +74,11 @@ private:
   vtkm::Int8 Value;
 };
 
+DeviceAdapterId make_DeviceAdapterId(vtkm::Int8 id)
+{
+  return DeviceAdapterId(id);
+}
+
 template <typename DeviceAdapter>
 struct DeviceAdapterTraits;
 }