layers:Update comments to avoid compiler warnings

The simple "// fall through" comment causes the compiler to omit a
warning.
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index d9b23da..f77af8f 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -1884,8 +1884,8 @@
                     return false;
                 }
             }
-            // Intentional fall-through to validate sampler
         }
+        // fall through
         case VK_DESCRIPTOR_TYPE_SAMPLER: {
             for (uint32_t di = 0; di < update->descriptorCount; ++di) {
                 if (!descriptors_[index + di].get()->IsImmutableSampler()) {