Fix IRGen to pass complete metadata to various concurrency builtins

Fixes rdar://146155888

lib/IRGen/GenMeta.cpp

@@ -6771,7 +6771,8 @@ namespace {
         return;
       }
 
-      // Emit a reference to the superclass.
+      // Emit a reference to the superclass. This should be abstract for now,
+      // but transitively completing the class will complete it.
       auto superclass = IGF.emitAbstractTypeMetadataRef(
                           getSuperclassForMetadata(IGM, Target));