[VK] Fix EnabledDeviceExtensions redefinition and drop on macOS build (#1251)

Fixes a redefinition of `EnabledDeviceExtensions` in
`VulkanDevice::create` introduced by the parallel merge of #1177 and
#1195, plus a related silent drop of `VK_EXT_mesh_shader` when both
extensions are enabled.

Assisted by Claude Opus 4.7.

Author: alsepkow

lib/API/VK/Device.cpp

@@ -1315,22 +1315,19 @@ class VulkanDevice : public offloadtest::Device {
       Features.pNext = &MeshFeatures;
     }
 
-    DeviceInfo.enabledExtensionCount =
-        static_cast<uint32_t>(EnabledDeviceExtensions.size());
-    DeviceInfo.ppEnabledExtensionNames = EnabledDeviceExtensions.data();
-    DeviceInfo.pEnabledFeatures = &Features.features;
-    DeviceInfo.pNext = Features.pNext;
-
 #ifdef VK_EXT_SHADER_IMAGE_ATOMIC_INT64_EXTENSION_NAME
-    llvm::SmallVector<const char *, 1> EnabledDeviceExtensions;
     if (HasShaderImageAtomicInt64Ext &&
         FeaturesImageAtomicInt64.shaderImageInt64Atomics)
       EnabledDeviceExtensions.push_back(
           VK_EXT_SHADER_IMAGE_ATOMIC_INT64_EXTENSION_NAME);
-    DeviceInfo.enabledExtensionCount = EnabledDeviceExtensions.size();
-    DeviceInfo.ppEnabledExtensionNames = EnabledDeviceExtensions.data();
 #endif
 
+    DeviceInfo.enabledExtensionCount =
+        static_cast<uint32_t>(EnabledDeviceExtensions.size());
+    DeviceInfo.ppEnabledExtensionNames = EnabledDeviceExtensions.data();
+    DeviceInfo.pEnabledFeatures = &Features.features;
+    DeviceInfo.pNext = Features.pNext;
+
     VkDevice Device = VK_NULL_HANDLE;
     if (auto Err = VK::toError(
             vkCreateDevice(PhysicalDevice, &DeviceInfo, nullptr, &Device),