go/types: add debugging code to detect use of incomplete interfaces

The comment for phase 2 of checker.interfaceType (typexpr.go:517)
requires that embedded interfaces be complete for correctness of
the algorithm.

Yet, the very next comment (typexpr.go:530) states that underlying
embedded interfaces may in fact be incomplete.

This is in fact the case and the underlying bug in issue #18395.

This change makes sure that new interface types are marked complete
when finished (per the implicit definition in Interface.Complete,
type.go:302). It also adds a check, enabled in debug mode only, to
detect the use of incomplete embedded interfaces during construction
of a new interface. In debug mode, this check fails for the testcase
in the issue (and several others).

This change has no noticeable impact with debug mode disabled.

For #18395.

Change-Id: Ibb81e47257651282fb3755a80a36ab5d392e636d
Reviewed-on: https://go-review.googlesource.com/78955


Reviewed-by: Alan Donovan <adonovan@google.com>