go/types, types2: misleading error message when type param is not comparable

Consider the following invalid program:
```
package p

func f[T any](t T) func(interface{}) bool {
	return func(x interface{}) bool {
		return x == t
	}
}
```

The type checker produces the error `cannot compare x == t (operator == not defined for interface{})`, but that's not accurate. The real problem is that `T` is not comparable.

CC @griesemer 