[Type Checker] Introduce a request for “overridden declarations”.

Introduce a new request kind to capture the computation of the set of
overridden declarations of a given declaration, eliminating the
stateful “setOverriddenDecls()” calls from the type checker.

lib/Serialization/Deserialization.cpp

@@ -2709,7 +2709,7 @@ ModuleFile::getDeclCheckedImpl(DeclID DID, Optional<DeclContext *> ForcedContext
         overriddenAssocTypes.push_back(overriddenAssocType);
       }
     }
-    (void)assocType->setOverriddenDecls(overriddenAssocTypes);
+    assocType->setOverriddenDecls(overriddenAssocTypes);
 
     break;
   }