Set the completed flag in NamedConstraint after the defn is complete (#6304)

A named constraint can not be
[identified](https://github.com/carbon-language/carbon-lang/blob/656150593c1e3fc2b6ccd83c7256a61e4bd04030/proposals/p5168.md#proposed-rules)
until its definition is complete, so this flag will be used to determine
if the named constraint is ready to be identified.

Author: danakj

toolchain/check/handle_named_constraint.cpp

@@ -156,8 +156,8 @@ auto HandleParseNode(Context& context,
           .Pop<Parse::NodeKind::NamedConstraintDefinitionStart>();
   context.inst_block_stack().Pop();
 
-  const auto& constraint_info =
-      context.named_constraints().Get(named_constraint_id);
+  auto& constraint_info = context.named_constraints().Get(named_constraint_id);
+  constraint_info.complete = true;
 
   // TODO: Do something with `require` and `alias` statements in the body of the
   // constraint.