[CUDA] Define __CUDACC__ before standard library headers

[tambre]

Summary

libstdc++ since version 7 when GNU extensions are enabled (e.g. -std=gnu++11) use it to avoid defining overloads using __float128.
This fixes compiling with GNU extensions failing due to __float128 being used.

Discovered at https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4442#note_737136.

[tra]

clang/lib/Headers/__clang_cuda_runtime_wrapper.h

44

We should undef CUDACC here to make sure it does not affect anything else in the headers we include further down.

[tra]

Undef CUDACC after the standard headers.

clang/lib/Headers/__clang_cuda_runtime_wrapper.h

 /*===---- __clang_cuda_runtime_wrapper.h - CUDA runtime support -------------===
  *
  * 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 __CLANG_CUDA_RUNTIME_WRAPPER_H__
 #define __CLANG_CUDA_RUNTIME_WRAPPER_H__
 
 #if defined(__CUDA__) && defined(__clang__)
 
 // Include some forward declares that must come before cmath.
 #include <__clang_cuda_math_forward_declares.h>
 
+// Define __CUDACC__ early as libstdc++ standard headers with GNU extensions
+// enabled depend on it to avoid using __float128, which is unsupported in
+// CUDA.
+#define __CUDACC__
+
 // Include some standard headers to avoid CUDA headers including them
 // while some required macros (like __THROW) are in a weird state.
 #include <cmath>
 #include <cstdlib>
 #include <stdlib.h>
 
 // Preserve common macros that will be changed below by us or by CUDA
 // headers.
 #pragma push_macro("__THROW")
 #pragma push_macro("__CUDA_ARCH__")
 
 // WARNING: Preprocessor hacks below are based on specific details of
 // CUDA-7.x headers and are not expected to work with any other
 // version of CUDA headers.