Make Win Clang ASan builds work

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 7d0b7d232f5237184d1aedd2577c075e502745fc..9a80c38ccfd917ff69f07935b96ee8ff43dcae4e 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -285,6 +285,9 @@ template("msvc_toolchain") {
       if (defined(invoker.is_component_build)) {
         is_component_build = invoker.is_component_build
       }
+      if (defined(invoker.is_asan)) {
+        is_asan = invoker.is_asan
+      }
 
       # This value needs to be passed through unchanged.
       host_toolchain = host_toolchain
@@ -369,6 +372,9 @@ template("win_x64_toolchains") {
     if (defined(invoker.is_component_build)) {
       is_component_build = invoker.is_component_build
     }
+
+    # ASan doesn't yet work w/ 64-bit builds.
+    is_asan = false
   }
 
   msvc_toolchain("clang_" + target_name) {
@@ -378,6 +384,10 @@ template("win_x64_toolchains") {
     cl = "${goma_prefix}$prefix/${clang_cl}"
     toolchain_os = "win"
     is_clang = true
+
+    # ASan doesn't yet work w/ 64-bit builds.
+    is_asan = false
+
     sys_include_flags = "${x64_toolchain_data.include_flags}"
 
     # This shares a name with a global and forward_variables_from won't clobber