[VK] Fix EnabledDeviceExtensions redefinition and silent extension drop

Two local variables both named `EnabledDeviceExtensions` were declared in
the same enclosing scope of `VulkanDevice::create`: one added by #1177
(mesh shader) and one added inside
`#ifdef VK_EXT_SHADER_IMAGE_ATOMIC_INT64_EXTENSION_NAME` by #1195
(InterlockedOr). Any platform whose Vulkan SDK defines that macro rejects
the file. CI surfaced it first on the macOS runner (Vulkan 1.4.335 +
clang) because that job ran ahead of the Windows jobs and short-circuited
the matrix, but Windows is equally affected: Windows VK CI uses Vulkan
SDK 1.4.350.0, and a local Windows + MSVC + Vulkan SDK 1.4.341.1 build
reproduces the failure as `error C2371: 'EnabledDeviceExtensions':
redefinition`.

Beyond the build break, the inner block also overwrote
`DeviceInfo.enabledExtensionCount` and `ppEnabledExtensionNames` that the
mesh-shader block had populated, silently dropping `VK_EXT_mesh_shader`
whenever both extensions were available. Neither bug existed in either
PR in isolation; both are post-merge artifacts.

Append the AtomicInt64 extension to the single `EnabledDeviceExtensions`
vector and assign the `DeviceInfo` extension fields once, after both
blocks.

Assisted-by: Claude Opus 4.7 (claude-opus-4.7-high)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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),