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

In asan builds, a few translation units still fall back to CL, see
http://llvm.org/PR24167

Also update the gn files to pass /WX (warnings-as-errors) in 32-bit non-asan
clang builds.

BUG=82385
TBR=rnk

Review URL: https://codereview.chromium.org/1229083005.

Cr-Original-Commit-Position: refs/heads/master@{#339309}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 28d32d89e5081223fab2982dc97252fd5150e721

common.gypi

@@ -5744,6 +5744,15 @@
                 ],
               },
             }],
+            ['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': [

config/compiler/BUILD.gn

@@ -127,6 +127,10 @@ config("compiler") {
       } else {
         cflags += [ "-m64" ]
       }
+      # TODO(thakis): Remove this once llvm.org/PR24167 is fixed.
+      if (is_asan) {
+        cflags += [ "/fallback" ]
+      }
       if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
           "True") {
         cflags += [
@@ -699,7 +703,8 @@ config("runtime_library") {
 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.
   }