test: add some test cases that were miscompiled by gccgo

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/40310043

test/fixedbugs/bug480.dir/a.go

+// Copyright 2013 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 a
+
+type S interface{
+	F() T
+}
+
+type T struct {
+	S
+}
+
+type U struct {
+	error
+}

test/fixedbugs/bug480.dir/b.go

+// Copyright 2013 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 b
+
+import "./a"
+
+var t a.T
+
+func F() error {
+	return a.U{}
+}

test/fixedbugs/bug480.go

+// compiledir
+
+// Copyright 2013 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.
+
+// Gccgo mishandled an import of a forward declared type.
+
+package ignored

test/fixedbugs/bug481.go

+// compile
+
+// Copyright 2013 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.
+
+// Returning an index into a conversion from string to slice caused a
+// compilation error when using gccgo.
+
+package p
+
+func F1(s string) byte {
+	return []byte(s)[0]
+}
+
+func F2(s string) rune {
+	return []rune(s)[0]
+}

test/fixedbugs/bug482.go

+// compile
+
+// Copyright 2013 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.
+
+// Using the same name for a field in a composite literal and for a
+// global variable that depends on the variable being initialized
+// caused gccgo to erroneously report "variable initializer refers to
+// itself".
+
+package p
+
+type S struct {
+	F int
+}
+
+var V = S{F: 1}
+
+var F = V.F

test/fixedbugs/issue6789.dir/a.go

+// Copyright 2013 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 a
+
+type unexported struct {
+        a int
+        b bool
+}
+
+type Struct struct {
+        unexported
+}

test/fixedbugs/issue6789.dir/b.go

+// Copyright 2013 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
+
+import "./a"
+
+type s a.Struct
+
+func main() {
+}

test/fixedbugs/issue6789.go

+// rundir
+
+// Copyright 2013 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.
+
+// Issue 6789: gccgo failed to find the hash function for an
+// unexported struct embedded in an exported struct.
+
+package ignored