go/parser: don't do method receiver checks at parse-time
go/parser: don't do method receiver checks at parse-time

The ast and printer don't care, and go/types can provide
a better error message.

This change requires an update to the tests for go/types
(go.tools repo). CL forthcoming.

Fixes issue 8493.

LGTM=adonovan
R=rsc, adonovan
CC=golang-codereviews
https://codereview.appspot.com/123010044

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go