Store an InterfaceId and a SpecificId in AssociatedEntityType. (#5252)

Instead of storing a `TypeId` that always refer to a facet type that
always contains exactly a single interface, store the interface
directly.

Also improve stringification of `LookupImplWitness` and witness access
into it, switching to using newly-added functionality for stringifying
specific interfaces.

toolchain/check/interface.cpp

@@ -25,9 +25,10 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
     return SemIR::ErrorInst::SingletonInstId;
   }
 
-  // The interface type is the type of `Self`.
-  auto self_type_id =
-      context.insts().Get(interface_info.self_param_id).type_id();
+  // This associated entity is being declared as a member of the self specific
+  // of the interface.
+  auto interface_specific_id =
+      context.generics().GetSelfSpecific(interface_info.generic_id);
 
   // Register this declaration as declaring an associated entity.
   auto index = SemIR::ElementIndex(
@@ -36,7 +37,8 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
 
   // Name lookup for the declaration's name should name the associated entity,
   // not the declaration itself.
-  auto type_id = GetAssociatedEntityType(context, self_type_id);
+  auto type_id =
+      GetAssociatedEntityType(context, interface_id, interface_specific_id);
   return AddInst<SemIR::AssociatedEntity>(
       context, context.insts().GetLocId(decl_id),
       {.type_id = type_id, .index = index, .decl_id = decl_id});