[AMDGPU] Correctly use the auxiliary toolchain to include libc++ (#109366)

Summary:
Now that we have a functional build for `libc++` on the GPU, it will now
find the target specific headers in `include/amdgcn-amd-amdhsa`. This is
a problem for offloading via OpenMP because we need the CPU and GPU
headers to match exactly. All the other toolchains forward this
correctly except the AMDGPU OpenMP one, fix this by overriding it to use
the host toolchain instead of the device one, so the triple is not
returned as `amdgcn-amd-amdhsa`.

Author: jhuber6

clang/lib/Driver/ToolChains/AMDGPUOpenMP.cpp

@@ -120,6 +120,11 @@ AMDGPUOpenMPToolChain::GetCXXStdlibType(const ArgList &Args) const {
   return HostTC.GetCXXStdlibType(Args);
 }
 
+void AMDGPUOpenMPToolChain::AddClangCXXStdlibIncludeArgs(
+    const llvm::opt::ArgList &Args, llvm::opt::ArgStringList &CC1Args) const {
+  HostTC.AddClangCXXStdlibIncludeArgs(Args, CC1Args);
+}
+
 void AMDGPUOpenMPToolChain::AddClangSystemIncludeArgs(
     const ArgList &DriverArgs, ArgStringList &CC1Args) const {
   HostTC.AddClangSystemIncludeArgs(DriverArgs, CC1Args);

clang/lib/Driver/ToolChains/AMDGPUOpenMP.h

@@ -42,6 +42,9 @@ class LLVM_LIBRARY_VISIBILITY AMDGPUOpenMPToolChain final
                         Action::OffloadKind DeviceOffloadKind) const override;
   void addClangWarningOptions(llvm::opt::ArgStringList &CC1Args) const override;
   CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  void AddClangCXXStdlibIncludeArgs(
+      const llvm::opt::ArgList &Args,
+      llvm::opt::ArgStringList &CC1Args) const override;
   void
   AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                             llvm::opt::ArgStringList &CC1Args) const override;

clang/test/Driver/amdgpu-openmp-toolchain.c

@@ -77,3 +77,7 @@
 // RUN:   %clang -### --target=x86_64-unknown-linux-gnu -emit-llvm -S -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx803 -nogpulib %s 2>&1 | FileCheck %s --check-prefix=CHECK-WARN-ATOMIC
 // CHECK-WARN-ATOMIC-NOT: "-cc1" "-triple" "x86_64-unknown-linux-gnu"{{.*}}"-Werror=atomic-alignment"
 // CHECK-WARN-ATOMIC: "-cc1" "-triple" "amdgcn-amd-amdhsa"{{.*}}"-Werror=atomic-alignment"
+
+// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -emit-llvm -S -fopenmp --offload-arch=gfx803 \
+// RUN:     -stdlib=libc++ -nogpulib %s 2>&1 | FileCheck %s --check-prefix=LIBCXX
+// LIBCXX-NOT: include/amdgcn-amd-amdhsa/c++/v1