[iOS/GN] Allow compilation with system clang.

build/config/ios/BUILD.gn

Index: build/config/ios/BUILD.gn
diff --git a/build/config/ios/BUILD.gn b/build/config/ios/BUILD.gn
index 3b9f846076dd723aa1820e95b42c7ef87e539044..75e740f4543e4c6412326787dca0fe8508c566a7 100644
--- a/build/config/ios/BUILD.gn
+++ b/build/config/ios/BUILD.gn
@@ -2,8 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/sysroot.gni")
 import("//build/config/ios/ios_sdk.gni")
+import("//build/config/sysroot.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
@@ -26,4 +27,13 @@ config("runtime_library") {
   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),
+    ]
+  }
 }