cmd/gc: disallow switch _ := v.(type)

Fixes #2827.

R=ken2
CC=golang-dev
https://golang.org/cl/5638045
diff --git a/test/typeswitch3.go b/test/typeswitch3.go
index 0789801..e11da7d 100644
--- a/test/typeswitch3.go
+++ b/test/typeswitch3.go
@@ -30,6 +30,10 @@
 	switch r.(type) {
 	case io.Writer:
 	}
+	
+	// Issue 2827.
+	switch _ := r.(type) {  // ERROR "invalid variable name _"
+	}
 }