For Go 1.10, works around a go/types bug that can't typecheck
a corner-case type cycle. Once we are confident that bugs like
this are gone from go/types then we can stop ignoring these
failures.

For #22890.

Change-Id: I38da57e01a0636323e1af4484c30871786125df3
Reviewed-on: https://go-review.googlesource.com/81500
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>