vulkan.h: Change return types of some functions to void

Functions that should be thought of as "impossible to fail in the face
of valid parameters" have had their return types changed to void.
This includes all of the vkDestroy functions, vkFreeMemory, and vkUnmapMemory.
vkUpdateDescriptorSets is also included, because of the frequency the function
is expected to be called.
diff --git a/layers/multi.cpp b/layers/multi.cpp
index cd4e94f..47b1ba5 100644
--- a/layers/multi.cpp
+++ b/layers/multi.cpp
@@ -40,16 +40,15 @@
 /******************************** Layer multi1 functions **************************/
 
 /* hook DestroyDevice to remove tableMap entry */
-VK_LAYER_EXPORT VkResult VKAPI multi1DestroyDevice(VkDevice device)
+VK_LAYER_EXPORT void VKAPI multi1DestroyDevice(VkDevice device)
 {
     VkLayerDispatchTable *pDisp = get_dispatch_table(multi1_device_table_map, device);
     dispatch_key key = get_dispatch_key(device);
 
     printf("At start of multi1 layer vkDestroyDevice()\n");
-    VkResult res = pDisp->DestroyDevice(device);
+    pDisp->DestroyDevice(device);
     multi1_device_table_map.erase(key);
     printf("Completed multi1 layer vkDestroyDevice()\n");
-    return res;
 }
 
 VK_LAYER_EXPORT VkResult VKAPI multi1CreateSampler(VkDevice device, const VkSamplerCreateInfo* pCreateInfo, VkSampler* pSampler)
@@ -144,16 +143,15 @@
 }
 
 /* hook DestroyInstance to remove tableInstanceMap entry */
-VK_LAYER_EXPORT VkResult VKAPI multi2DestroyInstance(VkInstance instance)
+VK_LAYER_EXPORT void VKAPI multi2DestroyInstance(VkInstance instance)
 {
     VkLayerInstanceDispatchTable *pDisp = get_dispatch_table(multi2_instance_table_map, instance);
     dispatch_key key = get_dispatch_key(instance);
 
     printf("At start of wrapped multi2 vkDestroyInstance()\n");
-    VkResult res = pDisp->DestroyInstance(instance);
+    pDisp->DestroyInstance(instance);
     multi2_instance_table_map.erase(key);
     printf("Completed multi2 layer vkDestroyInstance()\n");
-    return res;
 }
 
 VK_LAYER_EXPORT PFN_vkVoidFunction VKAPI multi2GetInstanceProcAddr(VkInstance inst, const char* pName)