[Clang] Disable `libc` headers for offloading languages

[jhuber6]

Summary

These headers are currently broken when included from the offloading
languages like OpenMP, OpenCL, CUDA, and HIP. Turn this logic off so we
can compile these languages when the GPU libc is installed. I am
currently trying to remedy this and have made an RFC for it in libc,
see https://discourse.llvm.org/t/rfc-implementing-gpu-headers-in-the-llvm-c-library/71523.

[JonChesterfield]

Thanks, unblocked :)

clang/lib/Driver/ToolChains/Clang.cpp

void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
   // from e.g. /usr/local/include.
   if (JA.isOffloading(Action::OFK_Cuda))
     getToolChain().AddCudaIncludeArgs(Args, CmdArgs);
   if (JA.isOffloading(Action::OFK_HIP))
     getToolChain().AddHIPIncludeArgs(Args, CmdArgs);
 
   // If we are compiling for a GPU target we want to override the system headers
   // with ones created by the 'libc' project if present.
+  // FIXME: We need to find a way to make these headers compatible with the
+  // host environment so they can be included from offloading languages. For now
+  // these are only active when targeting the GPU with cross-compilation.
   if (!Args.hasArg(options::OPT_nostdinc) &&
       !Args.hasArg(options::OPT_nogpuinc) &&
       !Args.hasArg(options::OPT_nobuiltininc) &&
+      C.getActiveOffloadKinds() == Action::OFK_None &&
       (getToolChain().getTriple().isNVPTX() ||
        getToolChain().getTriple().isAMDGCN())) {
 
       // Add include/gpu-none-libc/* to our system include path. This lets us use
       // GPU-specific system headers first. 
-      // TODO: We need to find a way to make these headers compatible with the
-      // host environment.
       SmallString<128> P(llvm::sys::path::parent_path(D.InstalledDir));
       llvm::sys::path::append(P, "include");
       llvm::sys::path::append(P, "gpu-none-llvm");
       CmdArgs.push_back("-c-isystem");
       CmdArgs.push_back(Args.MakeArgString(P));
   }
 
   // If we are offloading to a target via OpenMP we need to include the

clang/test/Driver/gpu-libc-headers.c

 // REQUIRES: nvptx-registered-target
 // REQUIRES: amdgpu-registered-target
 
-// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp --sysroot=./ \
-// RUN:     -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa --offload-arch=gfx908  \
-// RUN:     -nogpulib %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS
-// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp --sysroot=./ \
-// RUN:     -fopenmp-targets=nvptx64-nvidia-cuda -Xopenmp-target=nvptx64-nvidia-cuda --offload-arch=sm_70  \
-// RUN:     -nogpulib %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS
-// RUN:   %clang -### --target=nvptx64-nvidia-cuda -march=sm_70 -nogpulib --sysroot=./ %s 2>&1 | \
-// RUN:     FileCheck %s --check-prefix=CHECK-HEADERS
-// RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib --sysroot=./ %s 2>&1 | \
-// RUN:     FileCheck %s --check-prefix=CHECK-HEADERS
-// CHECK-HEADERS: "-cc1"{{.*}}"-c-isystem" "{{.*}}include{{.*}}gpu-none-llvm"{{.*}}"-isysroot" "./"
-
 // RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
 // RUN:     -nogpuinc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
 // RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
 // RUN:     -nostdinc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
 // RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
 // RUN:     -nobuiltininc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
 // CHECK-HEADERS-DISABLED-NOT: "-cc1"{{.*}}"-c-isystem" "{{.*}}include{{.*}}gpu-none-llvm"