If a name is not found in a class, perform lookup into base classes. (#3502)

This builds out a little infrastructure for one name scope to `extend`
another. We'll need more refinement here to cover other cases, but this
should provide some foundation for that future work.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/decl_name_stack.cpp

@@ -146,7 +146,7 @@ auto DeclNameStack::UpdateScopeIfNeeded(NameContext& name_context) -> void {
       name_context.state = NameContext::State::Resolved;
       name_context.target_scope_id = scope_id;
       context_->PushScope(name_context.resolved_inst_id, scope_id,
-                          context_->name_scopes().Get(scope_id).has_load_error);
+                          context_->name_scopes().Get(scope_id).has_error);
       break;
     }
     default: