cmd/gc: blank methods are not permitted in interface types

Fixes #6606.

LGTM=rsc
R=rsc
CC=golang-codereviews, gri
https://golang.org/cl/156210044
diff --git a/test/interface/explicit.go b/test/interface/explicit.go
index 36fa1a4..b10d02f 100644
--- a/test/interface/explicit.go
+++ b/test/interface/explicit.go
@@ -83,12 +83,12 @@
 
 
 type B1 interface {
-	_()
+	_() // ERROR "methods must have a unique non-blank name"
 }
 
 type B2 interface {
 	M()
-	_()
+	_() // ERROR "methods must have a unique non-blank name"
 }
 
 type T2 struct{}