[iOS/GN] Allow compilation with system clang.

build/config/ios/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/ios/ios_sdk.gni")
 import("//build/config/sysroot.gni")
-import("//build/config/ios/ios_sdk.gni")
+import("//build/toolchain/toolchain.gni")
 
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is iOS-only. Please see that target for advice on what should go in
 # :runtime_library vs. :compiler.
 config("runtime_library") {
   common_flags = [
     "-isysroot",
     sysroot,
 
     "-stdlib=libc++",
   ]
 
   if (use_ios_simulator) {
     common_flags += [ "-mios-simulator-version-min=$ios_deployment_target" ]
   } else {
     common_flags += [ "-miphoneos-version-min=$ios_deployment_target" ]
   }
 
   asmflags = common_flags
   cflags = common_flags
   ldflags = common_flags
+
+  # TODO(ios): Remove once Xcode's libc++ has LLVM r256325.
+  if (use_xcode_clang) {
+    cflags += [
+      "-isystem",
+      rebase_path("//third_party/llvm-build/Release+Asserts/include/c++/v1",
+                  root_build_dir),
+    ]
+  }
 }