[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/cmake/LibcudacxxBuildCompilerTargets.cmake

@@ -0,0 +1,53 @@
+# This file defines the `libcudacxx_build_compiler_targets()` function, which
+# creates the following interface targets:
+#
+# libcudacxx.compiler_interface
+# - Interface target linked into all targets in the libcudacxx developer build.
+#   Defines common warning flags, definitions, etc, including those defined in
+#   the global CCCL targets.
+
+cccl_get_libcudacxx()
+
+function(libcudacxx_build_compiler_targets)
+  set(cuda_compile_options)
+  set(cxx_compile_options)
+  set(cxx_compile_definitions)
+
+  #  if (CCCL_USE_LIBCXX)
+  #    list(APPEND cxx_compile_options "-stdlib=libc++")
+  #    list(APPEND cxx_compile_definitions "_ALLOW_UNSUPPORTED_LIBCPP=1")
+  #  endif()
+
+  # Set test specific flags
+  list(APPEND cxx_compile_definitions "CCCL_ENABLE_ASSERTIONS")
+  list(APPEND cxx_compile_definitions "CCCL_IGNORE_DEPRECATED_CPP_DIALECT")
+  list(
+    APPEND cxx_compile_definitions
+    "CCCL_IGNORE_DEPRECATED_DISCARD_MEMORY_HEADER"
+  )
+  list(
+    APPEND cxx_compile_definitions
+    "CCCL_IGNORE_DEPRECATED_STREAM_REF_HEADER"
+  )
+
+  if (CCCL_ENABLE_TILE)
+    list(APPEND cuda_compile_options "--enable-tile")
+  endif()
+
+  cccl_build_compiler_interface(
+    libcudacxx.compiler_flags
+    "${cuda_compile_options}"
+    "${cxx_compile_options}"
+    "${cxx_compile_definitions}"
+  )
+
+  add_library(libcudacxx.compiler_interface INTERFACE)
+  target_link_libraries(
+    libcudacxx.compiler_interface
+    INTERFACE
+      # order matters here, we need the libcudacxx options to override the cccl options.
+      cccl.compiler_interface
+      libcudacxx.compiler_flags
+      libcudacxx::libcudacxx
+  )
+endfunction()