Check root archetypes in a few more places.

This makes more code agnostic to the presence of NestedArchetypeType.

lib/IDE/CodeCompletion.cpp

@@ -2442,7 +2442,8 @@ public:
       // and not both.
       if (auto *archetypeType = t->getAs<ArchetypeType>()) {
         // Don't erase opaque archetype.
-        if (isa<OpaqueTypeArchetypeType>(archetypeType))
+        if (isa<OpaqueTypeArchetypeType>(archetypeType) &&
+            archetypeType->isRoot())
           return t;
 
         auto protos = archetypeType->getConformsTo();