Associated constants can be used in member function signatures (#5089)

This required allowing incomplete facet types where previously
completeness was required. Once we support named constraints, we will
need a way to consistently go from an interface to a facet type witness
index without requiring the interface to be complete in these cases.

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Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/import_ref.cpp

@@ -2314,6 +2314,7 @@ static auto AddInterfaceDefinition(ImportContext& context,
       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);