Force -Wno-shadow to avoid variable shadowing warnings.

Long term, ideally, these would be fixed and this flag can be removed.
For now, this is an expedient way to allow enabling -Wshadow in
Chromium.

Bug: chromium:794619
Change-Id: I038e5fc758fbb8284bb28cb4f7f8a4c44da0db66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3173891
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76996}

BUILD.gn

@@ -1212,7 +1212,12 @@ config("toolchain") {
   }
 
   if (is_clang) {
-    cflags += [ "-Wmissing-field-initializers" ]
+    cflags += [
+      "-Wmissing-field-initializers",
+
+      # TODO(v8:12245): Fix shadowing instances and remove.
+      "-Wno-shadow",
+    ]
 
     if (v8_current_cpu != "mips" && v8_current_cpu != "mipsel") {
       # We exclude MIPS because the IsMipsArchVariant macro causes trouble.