go/types: report error when recognizing issue #18395.

go/types: report error when recognizing issue #18395.

The fix (CL 79575) for #18395 is too risky at this stage of the Go 1.10
release process.

Since issue #18395 is easily recognized (but not easily fixed), report
an error instead of silently continuing. This avoids inscrutable follow
on errors.

Also, make sure all empty interfaces are "completed", and adjust
printing code to report incomplete interfaces.

For #18395.

Change-Id: I7fa5f97ff31ac9775c9a6d318fce9f526b0350cd
Reviewed-on: https://go-review.googlesource.com/80455
Reviewed-by: Alan Donovan <adonovan@google.com>