[dev.typeparams] go/types: embedded type cannot be a (pointer to) a type parameter

This is a port of CL 337353 to go/types, adjusted for the error API and
to comment out a test for MethodSet.

Some nearby error messages that were using errorf rather than error were
also adjusted.

Fixes #43621

Change-Id: I28c9747e044ec7a2863f6890db69475fb8c29231
Reviewed-on: https://go-review.googlesource.com/c/go/+/339651


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>