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>

src/cmd/compile/internal/base/debug.go

@@ -31,6 +31,7 @@ type DebugFlags struct {
 	GCProg                int    `help:"print dump of GC programs"`
 	Gossahash             string `help:"hash value for use in debugging the compiler"`
 	InlFuncsWithClosures  int    `help:"allow functions with closures to be inlined" concurrent:"ok"`
+	InterfaceCycles       int    `help:"allow anonymous interface cycles"`
 	Libfuzzer             int    `help:"enable coverage instrumentation for libfuzzer"`
 	LocationLists         int    `help:"print information about DWARF location list creation"`
 	Nil                   int    `help:"print information about nil checks"`