[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/libcudacxx/test/support/test_convertible.h

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SUPPORT_TEST_CONVERTIBLE_H
+#define SUPPORT_TEST_CONVERTIBLE_H
+
+// "test_convertible<Tp, Args...>()" is a metafunction used to check if 'Tp'
+// is implicitly convertible from 'Args...' for any number of arguments,
+// Unlike 'std::is_convertible' which only allows checking for single argument
+// conversions.
+
+#include <cuda/std/type_traits>
+
+#include "test_macros.h"
+
+namespace detail
+{
+template <class Tp>
+TEST_FUNC void eat_type(Tp);
+
+template <class Tp, class... Args>
+TEST_FUNC constexpr auto test_convertible_imp(int) -> decltype(eat_type<Tp>({cuda::std::declval<Args>()...}), true)
+{
+  return true;
+}
+
+template <class Tp, class... Args>
+TEST_FUNC constexpr auto test_convertible_imp(long) -> bool
+{
+  return false;
+}
+} // namespace detail
+
+template <class Tp, class... Args>
+TEST_FUNC constexpr bool test_convertible()
+{
+  return detail::test_convertible_imp<Tp, Args...>(0);
+}
+
+#endif // SUPPORT_TEST_CONVERTIBLE_H