[torque] allow casting from MaybeObject

Bug: v8:7793
Change-Id: Ief280b44f4fe74e2b38fc649e4d61b5e4146234a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2204097
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67828}

src/builtins/cast.tq

@@ -90,7 +90,17 @@ macro Cast<A : type extends WeakHeapObject>(o: A|Object): A labels CastError {
   return %RawDownCast<A>(o);
 }
 
-macro Cast<A: type>(o: MaybeObject): A labels CastError;
+macro Cast<A : type extends Object>(implicit context: Context)(o: MaybeObject):
+    A labels CastError {
+  typeswitch (o) {
+    case (WeakHeapObject): {
+      goto CastError;
+    }
+    case (o: Object): {
+      return Cast<A>(o) otherwise CastError;
+    }
+  }
+}
 
 Cast<Undefined>(o: MaybeObject): Undefined labels CastError {
   if (TaggedNotEqual(o, Undefined)) goto CastError;