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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index f7b3a7594987043ebef1746eb7dbbb5d735f9116..ed29e60fe388a47dde5ed9aa941732bba3b1931c 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -127,6 +127,9 @@ config("compiler") {
       } else {
         cflags += [ "-m64" ]
       }
+      if (is_asan) {
+        cflags += [ "/fallback" ]
+      }
       if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
           "True") {
         cflags += [
@@ -699,7 +702,8 @@ 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.
   }