clang/win: Add back /fallback for asan builds for now.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 5726 matching lines @@
                   '-Qunused-arguments',  # http://crbug.com/504658
                   '-Wno-microsoft',  # http://crbug.com/505296
                   '-Wno-switch',  # http://crbug.com/505308
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-unused-function',  # http://crbug.com/505316
                   '-Wno-unused-value',  # http://crbug.com/505318
                   '-Wno-unused-local-typedef',  # http://crbug.com/411648
                 ],
               },
             }],
+            ['clang==1 and target_arch=="ia32" and asan==1', {
+              # TODO(thakis): Remove this block once llvm.org/PR24167 is fixed.
+              'VCCLCompilerTool': {
+                'WarnAsError': 'false',
+                'AdditionalOptions': [
+                  '/fallback',
+                ],
+              },
+            }],
             ['clang==1 and clang_use_chrome_plugins==1', {
               'VCCLCompilerTool': {
                 'AdditionalOptions': [
                   '<@(clang_chrome_plugins_flags)',
                 ],
               },
             }],
             ['clang==1 and MSVS_VERSION == "2013"', {
               'VCCLCompilerTool': {
                 'AdditionalOptions': [
@@ skipping 441 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }