We should not silently permit a named type to match an unnamed

type.  This is OK in a conversion but not in an assignment.

R=rsc
http://go/go-review/1016024

test/bugs/bug212.go

@@ -7,6 +7,6 @@
 package main
 type I int 
 type S struct { f map[I]int }
-var v1 = S{ make(map[int]int) }		// OK--names are ignored.
-var v2 map[I]int = map[int]int{}	// OK.
+var v1 = S{ make(map[int]int) }		// ERROR "cannot|illegal|incompatible|wrong"
+var v2 map[I]int = map[int]int{}	// ERROR "cannot|illegal|incompatible|wrong"
 var v3 = S{ make(map[uint]int) }	// ERROR "cannot|illegal|incompatible|wrong"