Bring back decl engine optimisation for already concrete decls

[xunilrj]

Bring back the optimization removed by #6532.

if type_mapping.source_ids_contains_concrete_type(ctx.engines)
            || !decl_engine.get(&self.id).is_concrete(ctx.engines)
        {
            let mut decl = (*decl_engine.get(&self.id)).clone();
            if decl.subst(type_mapping, ctx).has_changes() {
                Some(
                    decl_engine
                        .insert(decl, decl_engine.get_parsed_decl_id(&self.id).as_ref())
                        .with_parent(decl_engine, self.id.into()),
                )
            } else {
                None
            }

The problem seems to be is_concrete is not working 100% for functions/methods with generics, because parent´s generics are not inserted into the function/method type parameters Vec.