layers: Remove IMAGE_STATE::is_swapchain_image data member

Replace it with a method, IsSwapchainImage(), that just checks
at the swapchain handle is non-null.
diff --git a/layers/synchronization_validation.cpp b/layers/synchronization_validation.cpp
index fa1a4ea..4c39918 100644
--- a/layers/synchronization_validation.cpp
+++ b/layers/synchronization_validation.cpp
@@ -30,7 +30,7 @@
 
 static bool SimpleBinding(const IMAGE_STATE &image_state) {
     bool simple =
-        SimpleBinding(static_cast<const BINDABLE &>(image_state)) || image_state.is_swapchain_image || image_state.bind_swapchain;
+        SimpleBinding(static_cast<const BINDABLE &>(image_state)) || image_state.IsSwapchainImage() || image_state.bind_swapchain;
 
     // If it's not simple we must have an encoder.
     assert(!simple || image_state.fragment_encoder.get());