cmd/gc: disallow selectors to the blank identifier

Fixes #4941.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7415051
diff --git a/test/blank1.go b/test/blank1.go
index c6e038a..4edb2db 100644
--- a/test/blank1.go
+++ b/test/blank1.go
@@ -9,8 +9,13 @@
 
 package _	// ERROR "invalid package name _"
 
+var t struct {
+	_ int
+}
+
 func main() {
 	_()	// ERROR "cannot use _ as value"
 	x := _+1	// ERROR "cannot use _ as value"
 	_ = x
+	_ = t._ // ERROR "cannot refer to blank field"
 }