go/types: slightly relax notion of structural type
This is a port of CL 363075 from types2 to go/types, adjusted for the
different error reporting API, and to adjust positions of error messages
in tests.
Change-Id: Ic6bfedf1152eff94bad20725b56e6ba804b2e3e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/363991
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>