go/types, types2: nil underlying for selectors on the current type decl

The following code crashes the type checker with a "nil underlying" panic, because we try to check the underlying when selection fails:

```
type X X.t
```
https://go.dev/play/p/H3Dv94uxQL9?v=gotip

Marking as a release blocker as this is a crash.

CC @griesemer 