Reset ArrayPortalToken when Detach is called

When `ArrayPortalToken::Detach` was called, the contained `Token` was
detached, but `ArrayPortal` being wrapped might also be holding its own
portal that it is decorating. To ensure that any dependent portals are
also detached, `ArrayPortalToken::Detach` resets itself.

vtkm/cont/internal/ArrayPortalToken.h

@@ -91,7 +91,14 @@ public:
   ///
   /// This will open up the `ArrayHandle` for reading and/or writing.
   ///
-  VTKM_CONT void Detach() { this->Token->DetachFromAll(); }
+  VTKM_CONT void Detach()
+  {
+    this->Token->DetachFromAll();
+
+    // Reset this portal in case the superclass is holding other array portals with their own
+    // tokens. Detach is supposed to invalidate the array portal, so it is OK to do this.
+    *this = ArrayPortalToken<PortalType_>();
+  }
 
   /// \brief Get the `Token` of the `ArrayPortal`.
   ///

vtkm/cont/testing/UnitTestArrayHandleVirtual.cxx

@@ -217,6 +217,15 @@ struct Test
     // the concreteArray getting locked up.
     CheckPortal(virtualArray.ReadPortal());
     SetPortal(concreteArray.WritePortal());
+
+    // Make sure Detach also correctly unlocks the concrete array.
+    auto readPortal = virtualArray.ReadPortal();
+    readPortal.Detach();
+    concreteArray.WritePortal();
+
+    auto writePortal = virtualArray.WritePortal();
+    writePortal.Detach();
+    concreteArray.WritePortal();
   }
 
   void operator()()