layers: Unify invalid command buffer handling

Created a single vector to track any cmd buffer dependencies that are destroyed
and thereby cause the cmd buffer to become invalid. Currently just tracking
the previous object types of descriptor sets and framebuffers, but will update
the tracking to include all cmd buffer dependencies in future commits.
diff --git a/layers/descriptor_sets.h b/layers/descriptor_sets.h
index 9282459..609f5d0 100644
--- a/layers/descriptor_sets.h
+++ b/layers/descriptor_sets.h
@@ -286,6 +286,7 @@
 class DescriptorSet : public BASE_NODE {
   public:
     using BASE_NODE::in_use;
+    using BASE_NODE::cb_bindings;
     DescriptorSet(const VkDescriptorSet, const DescriptorSetLayout *, const core_validation::layer_data *);
     ~DescriptorSet();
     // A number of common Get* functions that return data based on layout from which this set was created
@@ -331,11 +332,11 @@
     const DescriptorSetLayout *GetLayout() const { return p_layout_; };
     VkDescriptorSet GetSet() const { return set_; };
     // Return unordered_set of all command buffers that this set is bound to
-    std::unordered_set<GLOBAL_CB_NODE *> GetBoundCmdBuffers() const { return bound_cmd_buffers_; }
+    std::unordered_set<GLOBAL_CB_NODE *> GetBoundCmdBuffers() const { return cb_bindings; }
     // Bind given cmd_buffer to this descriptor set
-    void BindCommandBuffer(GLOBAL_CB_NODE *cb_node) { bound_cmd_buffers_.insert(cb_node); }
-    // If given cmd_buffer is in the bound_cmd_buffers_ set, remove it
-    void RemoveBoundCommandBuffer(GLOBAL_CB_NODE *cb_node) { bound_cmd_buffers_.erase(cb_node); }
+    void BindCommandBuffer(GLOBAL_CB_NODE *cb_node) { cb_bindings.insert(cb_node); }
+    // If given cmd_buffer is in the cb_bindings set, remove it
+    void RemoveBoundCommandBuffer(GLOBAL_CB_NODE *cb_node) { cb_bindings.erase(cb_node); }
     VkSampler const *GetImmutableSamplerPtrFromBinding(const uint32_t index) const {
         return p_layout_->GetImmutableSamplerPtrFromBinding(index);
     };
@@ -359,7 +360,6 @@
     bool some_update_; // has any part of the set ever been updated?
     VkDescriptorSet set_;
     const DescriptorSetLayout *p_layout_;
-    std::unordered_set<GLOBAL_CB_NODE *> bound_cmd_buffers_;
     std::vector<std::unique_ptr<Descriptor>> descriptors_;
     // Ptr to device data used for various data look-ups
     const core_validation::layer_data *device_data_;