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

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")
 import("//build/config/chrome_build.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 109 matching lines @@
         cflags += [ "-fmsc-version=1800" ]
       } else if (visual_studio_version == "2015") {
         cflags += [ "-fmsc-version=1900" ]
       }
 
       if (current_cpu == "x86") {
         cflags += [ "-m32" ]
       } else {
         cflags += [ "-m64" ]
       }
+      if (is_asan) {
+        cflags += [ "/fallback" ]
+      }
       if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
           "True") {
         cflags += [
           # cmd.exe doesn't understand ANSI escape codes by default,
           # so only enable them if something emulating them is around.
           "-fansi-escape-codes",
         ]
       }
     }
   } else {
@@ skipping 552 matching lines @@
 }
 
 # default_warning_flags collects all warning flags that are used by default.
 # This is in a variable instead of a config so that it can be used in
 # both chromium_code and no_chromium_code.  This way these flags are guaranteed
 # to appear on the compile command line after -Wall.
 
 default_warning_flags = []
 default_warning_flags_cc = []
 if (is_win) {
-  if (!is_clang || current_cpu != "x86") {
+  # TODO(thakis): Make this unconditional once llvm.org/PR24167 is fixed.
+  if (!is_clang || !is_asan) {
     default_warning_flags += [ "/WX" ]  # Treat warnings as errors.
   }
 
   default_warning_flags += [
     # Warnings permanently disabled:
 
     # TODO(GYP) The GYP build doesn't have this globally enabled but disabled
     # for a bunch of individual targets. Re-enable this globally when those
     # targets are fixed.
     "/wd4018",  # Comparing signed and unsigned values.
@@ skipping 533 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }