go/types, types2: bad "invalid composite literal type" error for type parameter with pointer structural type

F below is valid non-generic code, so I'd expect G to compile successfully too. However, go/types and types2 currently reject it.

```
package p

func F() {
        type T = *struct{ f int }
        _ = []T{{f: 1}}
}

func G[T ~*struct{ f int }]() {
        _ = []T{{f: 1}}
}
```

/cc @griesemer @findleyr @ianlancetaylor 
