Revert "Roll Clang 218707:223109 (+r223211, +r223219, -r220714)" and follow-ups.

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 744 matching lines @@
         # http://crbug.com/255186
         "-Wno-deprecated-register",
 
         # TODO(thakis): This used to be implied by -Wno-unused-function,
         # which we no longer use. Check if it makes sense to remove
         # this as well. http://crbug.com/316352
         "-Wno-unneeded-internal-declaration",
 
         # TODO(thakis): Remove, http://crbug.com/263960
         "-Wno-reserved-user-defined-literal",
-
-        # TODO(hans): Get this cleaned up.
-        "-Wno-inconsistent-missing-override",
       ]
     }
     if (gcc_version >= 48) {
       cflags_cc += [
         # See comment for -Wno-c++11-narrowing.
         "-Wno-narrowing",
 
         # TODO(thakis): Remove, http://crbug.com/263960
         "-Wno-literal-suffix",
       ]
@@ skipping 201 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }