Cannot use define_class with a local class

[katzdm]

The following code should compile:

int main() {
    struct S;
    static_assert(is_type(define_class(^S, {})));

    [[maybe_unused]] S s;
    return 0;
}

It currently fails with a message indicating that S is an unknown type name. As detailed in P2996.md, the implementation of define_class is fairly hacky; it has to more or less "pretend to be the parser" and mess with scopes and declaration contexts as the parser otherwise would while parsing a class definition. Some nuance of how the parser does its job must be being overlooked.