test: add testcases for gccgo bug

Add a couple of testcase for a gccgo type checking bug.

Updates #31412.

Change-Id: I7a813dafde78e4add1432602d2af5fe879415e1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/171761
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue31412a.go

+// compile
+
+// Copyright 2019 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.
+
+// This code was incorrectly flagged as erroneous by gccgo.
+
+package main
+
+type Name string
+
+type EFunc func(int) int
+
+func Register(f EFunc, names ...Name) int {
+	return f(len(names))
+}
+
+const (
+	B Name = "B"
+)
+
+func RegisterIt() {
+	n := B + "Duck"
+	d := B + "Goose"
+	f := func(x int) int { return x + 9 }
+	Register(f, n, d)
+}
+
+func main() {
+	RegisterIt()
+}

test/fixedbugs/issue31412b.go

+// errorcheck
+
+// Copyright 2019 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.
+
+// This code was incorrectly accepted by gccgo.
+
+package main
+
+type N string
+type M string
+
+const B N = "B"
+const C M = "C"
+
+func main() {
+	q := B + C // ERROR "mismatched types|incompatible types"
+	println(q)
+}