cmd/compile: misleading error trying to convert to pointer to generic

For invalid conversions such as 

```
func foo[A [1]int]() {
	var s string
	_ = (*A)(s)
}
```

the compiler reports

```
/tmp/qux.go:5:11: cannot convert s (variable of type string) to type *A: string does not implement *A (type *A is pointer to interface, not interface)
```

but the issue isn't _really_ about interfaces.