[dev.typeparams] go/types: cleanups around receiver type checks

This is a port of CL 333770 to go/types, adjusted for the error
reporting API and to not support compiler error messages. An error
message changed (to 'invalid receiver type' from just 'invalid
receiver'), so a test had to be adjusted.

Change-Id: I166e8831d8c9f98ebfb0270fe5221586fc112825
Reviewed-on: https://go-review.googlesource.com/c/go/+/335079


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>