Handle read-only portals in ArrayHandleMultiplexer

The ArrayPortalMultiplexer always called Set on the delegate portals.
However, sometimes the portals were not supported. Instead, only call
when the delegate is supported.

Probably the "right" thing to do would be to check all posible portals
for support, but that sounds like it would slow down the compile.

vtkm/cont/ArrayHandleMultiplexer.h

@@ -10,6 +10,7 @@
 #ifndef vtk_m_cont_ArrayHandleMultiplexer_h
 #define vtk_m_cont_ArrayHandleMultiplexer_h
 
+#include <vtkm/Assert.h>
 #include <vtkm/TypeTraits.h>
 
 #include <vtkm/internal/Variant.h>
@@ -50,14 +51,36 @@ struct ArrayPortalMultiplexerGetFunctor
 
 struct ArrayPortalMultiplexerSetFunctor
 {
-  VTKM_SUPPRESS_EXEC_WARNINGS
   template <typename PortalType>
   VTKM_EXEC_CONT void operator()(const PortalType& portal,
                                  vtkm::Id index,
                                  const typename PortalType::ValueType& value) const noexcept
+  {
+    this->DoSet(
+      portal, index, value, typename vtkm::internal::PortalSupportsSets<PortalType>::type{});
+  }
+
+private:
+  VTKM_SUPPRESS_EXEC_WARNINGS
+  template <typename PortalType>
+  VTKM_EXEC_CONT void DoSet(const PortalType& portal,
+                            vtkm::Id index,
+                            const typename PortalType::ValueType& value,
+                            std::true_type) const noexcept
   {
     portal.Set(index, value);
   }
+
+  VTKM_SUPPRESS_EXEC_WARNINGS
+  template <typename PortalType>
+  VTKM_EXEC_CONT void DoSet(const PortalType&,
+                            vtkm::Id,
+                            const typename PortalType::ValueType&,
+                            std::false_type) const noexcept
+  {
+    // This is an error but whatever.
+    VTKM_ASSERT(false && "Calling Set on a portal that does not support it.");
+  }
 };
 
 } // namespace detail