go/types, types2: delay receiver type validation

Delay validation of receiver type as it may cause premature expansion
of types the receiver type is dependent on. This was actually a TODO.

While the diff looks large-ish, the actual change is small: all the
receiver validation code has been moved inside the delayed function
body, and a couple of comments have been adjusted.

Fixes #51232.
Fixes #51233.

Change-Id: I44edf0ba615996266791724b832d81b9ccb8b435
Reviewed-on: https://go-review.googlesource.com/c/go/+/387918


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