Refactor interface and impl NameScope importing to resemble classes (#5585)

Similar to the class change made in
c6f25e90185f497cfd1cddd5ef87be4aa19238b5 but without tests as it doesn't
appear that the same bug is reachable for these cases at the moment -
but seems good to match the approach in case cycles appear in the future
and to make the code consistent.

The impl case didn't seem to be able to use the new common utility
function, since it splits the two pieces of work and only does the
second conditionally.

toolchain/check/import_ref.cpp

@@ -1604,6 +1604,14 @@ static auto MakeIncompleteClass(ImportContext& context,
   return {class_decl.class_id, self_const_id};
 }
 
+static auto InitializeNameScopeAndImportRefs(
+    ImportContext& context, const SemIR::NameScope& import_scope,
+    SemIR::NameScope& new_scope, SemIR::InstId decl_id, SemIR::NameId name_id,
+    SemIR::NameScopeId parent_scope_id) {
+  new_scope.Set(decl_id, name_id, parent_scope_id);
+  AddNameScopeImportRefs(context, import_scope, new_scope);
+}
+
 // Fills out the class definition for an incomplete class.
 static auto AddClassDefinition(ImportContext& context,
                                const SemIR::Class& import_class,
@@ -1621,9 +1629,9 @@ static auto AddClassDefinition(ImportContext& context,
 
   // Push a block so that we can add scoped instructions to it.
   context.local_context().inst_block_stack().Push();
-  new_scope.Set(new_class.first_owning_decl_id, SemIR::NameId::None,
-                new_class.parent_scope_id);
-  AddNameScopeImportRefs(context, import_scope, new_scope);
+  InitializeNameScopeAndImportRefs(
+      context, import_scope, new_scope, new_class.first_owning_decl_id,
+      SemIR::NameId::None, new_class.parent_scope_id);
   new_class.body_block_id = context.local_context().inst_block_stack().Pop();
 
   if (import_class.base_id.has_value()) {
@@ -2008,6 +2016,8 @@ static auto MakeImplDeclaration(ImportContext& context,
         .constraint_id = SemIR::TypeInstId::None,
         .interface = SemIR::SpecificInterface::None,
         .witness_id = witness_id,
+        .scope_id = import_impl.is_complete() ? AddPlaceholderNameScope(context)
+                                              : SemIR::NameScopeId::None,
         .is_final = import_impl.is_final}});
 
   // Write the impl ID into the ImplDecl.
@@ -2024,9 +2034,9 @@ static auto AddImplDefinition(ImportContext& context,
   new_impl.defined = true;
 
   if (import_impl.scope_id.has_value()) {
-    new_impl.scope_id = context.local_name_scopes().Add(
-        new_impl.first_owning_decl_id, SemIR::NameId::None,
-        new_impl.parent_scope_id);
+    auto& new_scope = context.local_name_scopes().Get(new_impl.scope_id);
+    new_scope.Set(new_impl.first_owning_decl_id, SemIR::NameId::None,
+                  new_impl.parent_scope_id);
     // Import the contents of the definition scope, if we might need it. Name
     // lookup is never performed into this scope by a user of the impl, so
     // this is only necessary in the same library that defined the impl, in
@@ -2035,7 +2045,6 @@ static auto AddImplDefinition(ImportContext& context,
     // TODO: Check to see if this impl is owned by the API file, rather than
     // merely being imported into it.
     if (context.import_ir_id() == SemIR::ImportIRId::ApiForImpl) {
-      auto& new_scope = context.local_name_scopes().Get(new_impl.scope_id);
       const auto& import_scope =
           context.import_name_scopes().Get(import_impl.scope_id);
 
@@ -2161,7 +2170,9 @@ static auto MakeInterfaceDecl(ImportContext& context,
   interface_decl.interface_id = context.local_interfaces().Add(
       {GetIncompleteLocalEntityBase(context, interface_decl_id,
                                     import_interface),
-       {}});
+       {.scope_id = import_interface.is_complete()
+                        ? AddPlaceholderNameScope(context)
+                        : SemIR::NameScopeId::None}});
 
   if (import_interface.has_parameters()) {
     interface_decl.type_id = GetGenericInterfaceType(
@@ -2181,17 +2192,16 @@ static auto AddInterfaceDefinition(ImportContext& context,
                                    const SemIR::Interface& import_interface,
                                    SemIR::Interface& new_interface,
                                    SemIR::InstId self_param_id) -> void {
-  new_interface.scope_id = context.local_name_scopes().Add(
-      new_interface.first_owning_decl_id, SemIR::NameId::None,
-      new_interface.parent_scope_id);
   auto& new_scope = context.local_name_scopes().Get(new_interface.scope_id);
-  new_scope.set_is_interface_definition();
   const auto& import_scope =
       context.import_name_scopes().Get(import_interface.scope_id);
 
   // Push a block so that we can add scoped instructions to it.
   context.local_context().inst_block_stack().Push();
-  AddNameScopeImportRefs(context, import_scope, new_scope);
+  InitializeNameScopeAndImportRefs(
+      context, import_scope, new_scope, new_interface.first_owning_decl_id,
+      SemIR::NameId::None, new_interface.parent_scope_id);
+  new_scope.set_is_interface_definition();
   new_interface.associated_entities_id = AddAssociatedEntities(
       context, new_interface.scope_id, import_interface.associated_entities_id);
   new_interface.body_block_id =