api-query: Continue device initialization when a backend fails

Device::initialize() returns early when any backend fails to initialize,
preventing subsequent backends from being discovered. For example, a
Vulkan initialization failure (e.g. VK_ERROR_INCOMPATIBLE_DRIVER when
MoltenVK is not installed) blocks Metal device discovery on macOS.

Collect backend initialization errors with joinErrors() and return them
together instead. Also make lit.cfg.py resilient to an empty device list
from api-query.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MarijnS95

lib/API/Device.cpp

@@ -57,25 +57,27 @@ void Device::registerDevice(std::shared_ptr<Device> D) {
 }
 
 llvm::Error Device::initialize(const DeviceConfig Config) {
+  llvm::Error Result = llvm::Error::success();
 #ifdef OFFLOADTEST_ENABLE_D3D12
   if (auto Err = initializeDXDevices(Config))
-    return Err;
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
 #endif
 #ifdef OFFLOADTEST_ENABLE_VULKAN
   if (auto Err = initializeVulkanDevices(Config))
-    return Err;
-  // Validation layers have internal state which require a specific destruction
-  // ordering. Relying on the global dtor call for this is unreliable and can
-  // cause a null-deref in the validation layers during the final
-  // vkDestroyInstance. This is a known limitation of the validation layers
-  // which explicitely requires using atexit.
-  atexit(Device::cleanupVulkanDevices);
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
+  else
+    // Validation layers have internal state which require a specific
+    // destruction ordering. Relying on the global dtor call for this is
+    // unreliable and can cause a null-deref in the validation layers during the
+    // final vkDestroyInstance. This is a known limitation of the validation
+    // layers which explicitely requires using atexit.
+    atexit(Device::cleanupVulkanDevices);
 #endif
 #ifdef OFFLOADTEST_ENABLE_METAL
   if (auto Err = initializeMtlDevices(Config))
-    return Err;
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
 #endif
-  return llvm::Error::success();
+  return Result;
 }
 
 void Device::uninitialize() { DeviceContext::instance().unregisterDevices(); }

test/lit.cfg.py

@@ -216,7 +216,7 @@ def setDeviceFeatures(config, device, compiler):
 target_device = None
 # Find the right device to configure against
 pattern = re.compile(GPUName, re.IGNORECASE)
-for device in devices["Devices"]:
+for device in devices.get("Devices", []):
     is_warp = "Microsoft Basic Render Driver" in device["Description"]
     is_gpu_name_match = bool(pattern.search(device["Description"]))
     if device["API"] == "DirectX" and config.offloadtest_enable_d3d12: