go/types, types2: better error message for some invalid receiver errors (cleanup)

go/types, types2: better error message for some invalid receiver errors (cleanup)

Use the 1.17 compiler error message together with the receiver base type.
Also, simplify and flatten the receive testing logic for clarity.

Change-Id: I71e58f261900dd7a85d2eb89a310c36b68d1b0b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/396298
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>