[AMDGPU][OpenMP] Wrap amdgcn declare variant inside ifdef

[pdhaliwal]

Summary

This fixes the issue https://bugs.llvm.org/show_bug.cgi?id=51337

[JonChesterfield]

Yes, ship it. We'll look at fixing variant and pull the ifdef back out once it's working.

clang/lib/Headers/openmp_wrappers/__clang_openmp_device_functions.h

 /*===- __clang_openmp_device_functions.h - OpenMP device function declares -===
  *
  * 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_OPENMP_DEVICE_FUNCTIONS_H__
 #define __CLANG_OPENMP_DEVICE_FUNCTIONS_H__
 
 #ifndef _OPENMP
 #error "This file is for OpenMP compilation only."
 #endif
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 #pragma omp begin declare variant match(                                       \
     device = {arch(nvptx, nvptx64)}, implementation = {extension(match_any)})
 
 #define __CUDA__
 #define __OPENMP_NVPTX__
 
 /// Include declarations for libdevice functions.
 #include <__clang_cuda_libdevice_declares.h>
 
 /// Provide definitions for these functions.
 #include <__clang_cuda_device_functions.h>
 
 #undef __OPENMP_NVPTX__
 #undef __CUDA__
 
 #pragma omp end declare variant
 
+#ifdef __AMDGCN__
 #pragma omp begin declare variant match(device = {arch(amdgcn)})
 
 // Import types which will be used by __clang_hip_libdevice_declares.h
 #ifndef __cplusplus
 #include <stdbool.h>
 #include <stdint.h>
 #endif
 
 #define __OPENMP_AMDGCN__
 #pragma push_macro("__device__")
 #define __device__
 
 /// Include declarations for libdevice functions.
 #include <__clang_hip_libdevice_declares.h>
 
 #pragma pop_macro("__device__")
 #undef __OPENMP_AMDGCN__
 
 #pragma omp end declare variant
+#endif
 
 #ifdef __cplusplus
 } // extern "C"
 #endif
 
 // Ensure we make `_ZdlPv`, aka. `operator delete(void*)` available without the
 // need to `include <new>` in C++ mode.
 #ifdef __cplusplus