[iOS/GN] Allow compilation with system clang.

build/config/clang/BUILD.gn

Index: build/config/clang/BUILD.gn
diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
index dbff398e1f6d06231396b432f7167d00bfc50e38..ea568b3da47668d6bf3538e729cf66ab4f06f9d1 100644
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -60,6 +60,15 @@ config("find_bad_constructs") {
       ]
     }
   }
+
+  # TODO(ios): Remove once Xcode's libc++ has LLVM r256325.
+  if (use_system_clang && is_ios) {
+    cflags += [
+      "-isystem",
+      rebase_path("//third_party/llvm-build/Release+Asserts/include/c++/v1",
+                  root_build_dir),
+    ]
+  }
 }
 
 # Enables some extra Clang-specific warnings. Some third-party code won't