6g thinks a struct with one entry yields a composite that is a basic conversion

R=rsc
DELTA=12  (12 added, 0 deleted, 0 changed)
OCL=16989
CL=16998

test/bugs/bug112.go

+// $G $D/$F.go || echo BUG should compile
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type T struct { s string }
+var t = T{"hi"}
+
+func main() {}
+
+/*
+bug112.go:6: illegal conversion of constant to T
+*/