[libc] Prevent system headers from being included for the GPU build

It's very important that the GPU build does not include any system
directories. We currently use `-ffreestanding` to disable a lot of these
features, but we can still accidentally include them if they are not
provided by `libc` yet. This patch changes this to use `-nostdinc` to
disable all standard search paths. Then we use the compiler's resource
directory to pick up the provided headers like `stdint.h`.

Differential Revision: https://reviews.llvm.org/D159445

Author: jhuber6

libc/CMakeLists.txt

@@ -71,9 +71,13 @@ if(COMMAND_RETURN_CODE EQUAL 0)
   message(STATUS "Set COMPILER_RESOURCE_DIR to "
                  "${COMPILER_RESOURCE_DIR} using --print-resource-dir")
 else()
-  set(COMPILER_RESOURCE_DIR OFF)
-  message(STATUS "COMPILER_RESOURCE_DIR not set
-                  --print-resource-dir not supported by host compiler")
+  if (LIBC_TARGET_ARCHITECTURE_IS_GPU)
+    message(FATAL_ERROR "COMPILER_RESOURCE_DIR must be set for GPU builds")
+  else()
+    set(COMPILER_RESOURCE_DIR OFF)
+    message(STATUS "COMPILER_RESOURCE_DIR not set
+                    --print-resource-dir not supported by host compiler")
+  endif()
 endif()
 
 option(LLVM_LIBC_FULL_BUILD "Build and test LLVM libc as if it is the full libc" OFF)

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -68,6 +68,11 @@ function(_get_common_compile_options output_var flags)
   if (LIBC_TARGET_ARCHITECTURE_IS_GPU)
     list(APPEND compile_options "-nogpulib")
     list(APPEND compile_options "-fvisibility=hidden")
+
+    # Manually disable all standard include paths and include the resource
+    # directory to prevent system headers from being included.
+    list(APPEND compile_options "-isystem${COMPILER_RESOURCE_DIR}/include")
+    list(APPEND compile_options "-nostdinc")
   endif()
   set(${output_var} ${compile_options} PARENT_SCOPE)
 endfunction()