go/types, types2: refer to type parameter if so for interface pointer errors

go/types, types2: refer to type parameter if so for interface pointer errors

Follow-up on comment in CL 376914.

Also:
- add missing check != nil test in assignableTo
- use check.sprintf rather than fmt.Sprintf in missingMethodReason

For #48312.

Change-Id: Ie209b4101a7f2c279e42a59987d0068079c8b69f
Reviewed-on: https://go-review.googlesource.com/c/go/+/377375
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>