Revert "Roll Clang 214024:216630 (+216684) and switch to CMake (try 2)"

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 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/config/android/config.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
@@ skipping 91 matching lines @@
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
   }
 
   if (is_clang) {
     cflags += [
       # TODO(thakis): Remove once http://llvm.org/PR20354 is fixed
       # and the fix is in chromium.
       "-fno-slp-vectorize",
     ]
-
-    if (is_debug) {
-      # Allow comparing the address of references and 'this' against 0
-      # in debug builds. Technically, these can never be null in
-      # well-defined C/C++ and Clang can optimize such checks away in
-      # release builds, but they may be used in asserts in debug builds.
-      cflags_cc += [
-        "-Wno-undefined-bool-conversion",
-        "-Wno-tautological-undefined-compare",
-      ]
-    }
   }
 
   if (is_clang && !is_win) {
     # This is here so that all files get recompiled after a clang roll and
     # when turning clang on or off. (defines are passed via the command line,
     # and build system rebuild things when their commandline changes). Nothing
     # should ever read this define.
     defines += [
       "CR_CLANG_REVISION=" +
           exec_script(
@@ skipping 801 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }