gn: define and use clang_base_path

build/config/clang/clang.gni

 # 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/toolchain/toolchain.gni")
 
 declare_args() {
   # Indicates if the build should use the Chrome-specific plugins for enforcing
   # coding guidelines, etc. Only used when compiling with Clang.
   clang_use_chrome_plugins = is_clang && !is_nacl && !use_xcode_clang
+
+  clang_base_path = "//third_party/llvm-build/Release+Asserts"
 }