[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) {

[Nico, 2016/06/21 12:30:59]

can we call this use_xcode_clang please? i don't want people to try and add
"system clang" support for builds other than ios.

(also, have you experimented with shipping a chromium clang release? what are
blockers?)

[sdefresne, 2016/06/21 13:21:58]

I've renamed the variable to "is_clang_xcode" to be closer to the gyp name
("clang_xcode") while still following the same naming convention than "is_clang"
(named "clang" with gyp).

Regarding shipping a build of Chrome on iOS using ToT clang, if/when we do it we
want to decouple this from the transition from gyp to gn.

[justincohen, 2016/06/22 09:15:07]

We haven't tried shipping with a chromium clang.

We sometimes see features added to Xcode's clang that are required to support
some new new iOS thing.  While those features do eventually make it to chromium
clang, we'd have delay support.

I don't know of anything blocking us today, but who knows what the next Xcode
beta will include.

[Nico, 2016/06/22 14:30:27]

The last two times (bitcode and...uh, don't remember, there was one other thing.
arm64?) it took like a month, which doesn't seem so terrible.

+    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