go/types: check if the interface is already complete in Complete

Once Interfaces have been completed they must never be written again,
as they may be used concurrently.

Avoid writing Interface.complete unnecessarily in Complete. Also, update
documentation to reflect that Complete must be called before the
Interface may be considered safe for concurrent use.

For #47726

Change-Id: Ic9fd1395ab0dd6d3499f7a698dadf315abcddab8
Reviewed-on: https://go-review.googlesource.com/c/go/+/342749


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>