cmd/compile: reject anonymous interface cycles

This CL changes cmd/compile to reject anonymous interface cycles like:

	type I interface { m() interface { I } }

We don't anticipate any users to be affected by this change in
practice. Nonetheless, this CL also adds a `-d=interfacecycles`
compiler flag to suppress the error. And assuming no issue reports
from users, we'll move the check into go/types and types2 instead.

Updates #56103.

Change-Id: I1f1dce2d7aa19fb388312cc020e99cc354afddcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/445598
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue16369.go

@@ -1,4 +1,4 @@
-// compile
+// compile -d=interfacecycles
 
 // Copyright 2016 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style