[infra] Roll clang to match the version used by Flutter

tools/buildtools/linux64/clang-format

+#!/usr/bin/env python

[aam, 2017/05/09 01:49:17]

When invoked via 'git cl format' this script won't be able to find python in the
environment PATH because PATH is reset by git_cl.py
https://chromium.googlesource.com/chromium/tools/depot_tools.git/+/HEAD/git_c...

Easiest fix would be to do #!/usr/bin/python here instead.

+# Copyright 2017 The Dart project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Forwards to the clang-format for the Linux toolchain."""
+
+import os
+import subprocess
+import sys
+
+DART_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

[aam, 2017/05/09 01:23:52]

DART_ROOT here actually points to dart-sdk/sdk/buildtools instead of
dart-sdk/sdk. 
So perhaps these two lines should read 
BUILDTOOlS = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
TOOLCHAIN = os.path.join(BUILDTOOlS, 'toolchain')

+TOOLCHAIN = os.path.join(DART_ROOT, 'buildtools', 'toolchain')
+CLANG_FORMAT = os.path.join(TOOLCHAIN, 'clang+llvm-x86_64-linux', 'bin', 'clang-format')
+
+def main(argv):
+  return subprocess.call([CLANG_FORMAT] + argv[1:])
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))