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

@@ -15,7 +15,6 @@
 
 #include "llvm/Support/Error.h"
 
-#include <cstdlib>
 #include <memory>
 
 using namespace offloadtest;
@@ -27,21 +26,24 @@ Device::~Device() {}
 llvm::Expected<llvm::SmallVector<std::unique_ptr<Device>>>
 offloadtest::initializeDevices(const DeviceConfig Config) {
   llvm::SmallVector<std::unique_ptr<Device>> Devices;
+  llvm::Error Result = llvm::Error::success();
 
 #ifdef OFFLOADTEST_ENABLE_D3D12
   if (auto Err = initializeDX12Devices(Config, Devices))
-    return Err;
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
 #endif
 
 #ifdef OFFLOADTEST_ENABLE_VULKAN
   if (auto Err = initializeVulkanDevices(Config, Devices))
-    return Err;
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
 #endif
 
 #ifdef OFFLOADTEST_ENABLE_METAL
   if (auto Err = initializeMetalDevices(Config, Devices))
-    return Err;
+    Result = llvm::joinErrors(std::move(Result), std::move(Err));
 #endif
 
+  if (Result)
+    return std::move(Result);
   return Devices;
 }

test/lit.cfg.py

@@ -241,7 +241,7 @@ 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: