more interface embedding tests

6g's error for I4 is not quite accurate
(it calls I4 a non-interface) but also
not a crash or infinite loop.

R=iant
DELTA=15  (15 added, 0 deleted, 0 changed)
OCL=33701
CL=33709

test/fixedbugs/bug195.go

@@ -10,3 +10,18 @@ type I1 interface { I2 }	// ERROR "interface"
 type I2 int
 
 type I3 interface { int }	// ERROR "interface"
+
+type S struct {
+	x interface{ S }	// ERROR "interface"
+}
+type I4 interface {
+	I4	// ERROR "interface"
+}
+
+type I5 interface {
+	I6
+}
+
+type I6 interface {
+	I5	// ERROR "interface"
+}