test: add new test for gccgo compilation problem

New test for issue 30659 (compilation error due to bad
export data).

Updates #30659.

Change-Id: I2541ee3c379e5b22033fea66bb4ebaf720cc5e1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/166917
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue30659.dir/a.go

+// 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.
+
+package a
+
+type I interface {
+	I2
+}
+type I2 interface {
+	M()
+}
+type S struct{}
+
+func (*S) M() {}
+
+func New() I {
+	return &S{}
+}

test/fixedbugs/issue30659.dir/b.go

+// 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.
+
+package b
+
+import (
+	"./a"
+)
+
+func B(p1 a.I, p2 a.I2) int {
+	return 42
+}

test/fixedbugs/issue30659.go

+// compiledir
+
+// 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.
+
+package ignored