fix getVKAdapters on MoltenVK (shader-slang#4852)

Author: skallweitNV

tools/gfx/vulkan/vk-helper-functions.cpp

@@ -490,9 +490,15 @@ Result SLANG_MCALL getVKAdapters(List<AdapterInfo>& outAdapters)
         VkInstanceCreateInfo instanceCreateInfo = { VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO };
         const char* instanceExtensions[] = {
             VK_KHR_GET_PHYSICAL_DEVICE_PROPERTIES_2_EXTENSION_NAME,
+#if SLANG_APPLE_FAMILY
+            VK_KHR_PORTABILITY_ENUMERATION_EXTENSION_NAME,
+#endif
         };
         instanceCreateInfo.enabledExtensionCount = SLANG_COUNT_OF(instanceExtensions);
         instanceCreateInfo.ppEnabledExtensionNames = &instanceExtensions[0];
+#if SLANG_APPLE_FAMILY
+        instanceCreateInfo.flags = VK_INSTANCE_CREATE_ENUMERATE_PORTABILITY_BIT_KHR;
+#endif
         VkInstance instance;
         SLANG_VK_RETURN_ON_FAIL(api.vkCreateInstance(&instanceCreateInfo, nullptr, &instance));