cmd/compile/internal/types2: break cycles in invalid types

This CL reverses the change in CL 284254 (which was ported
to types2) which originated in CL 240901 to address a crash
in a test created by a fuzzer (that crash appears to be
avoided in other ways, now).

This exposed another bug in typeset.go where we don't look
for the underlying type when testing if a type is an interface
or not. Fixed that as well.

Adjusted a test case that now doesn't report an error anymore
(which is good).

Fixes #48819.

Change-Id: I611d68e053d6b8a2f7176d0cd5a44da2df28ad21
Reviewed-on: https://go-review.googlesource.com/c/go/+/354329
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/decl.go

@@ -358,6 +358,7 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 				if tn == t.obj {
 					check.cycleError(path[i:])
 					t.info = invalid
+					t.underlying = Typ[Invalid]
 					return t.info
 				}
 			}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue41124.go2

@@ -12,7 +12,7 @@ type Nat /* ERROR cycle */ interface {
 
 type Zero struct{}
 type Succ struct{
-	Nat // ERROR interface contains type constraints
+	Nat // Nat contains type constraints but is invalid, so no error
 }
 
 // Struct tests.

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48819.src

@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type T /* ERROR illegal cycle in declaration of T */ struct {
+	T
+}
+
+func _(t T) {
+	_ = unsafe.Sizeof(t) // should not go into infinite recursion here
+}

src/cmd/compile/internal/types2/typeset.go

@@ -294,7 +294,7 @@ func computeInterfaceTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *_
 			// Union parsing reports a (delayed) error, so we can ignore this entry.
 			continue
 		default:
-			if typ == Typ[Invalid] {
+			if u == Typ[Invalid] {
 				continue
 			}
 			if check != nil && !check.allowVersion(check.pkg, 1, 18) {