test: add test that failed with gccgo

Test with some code that triggered a compilation error bug in gccgo.

Updates #33866.

Change-Id: Ib2f226bbbebbfae33b41037438fe34dc5f2ad034
Reviewed-on: https://go-review.googlesource.com/c/go/+/193261
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue33866.dir/a.go

@@ -0,0 +1,18 @@
+// 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 Builder struct {
+	x int
+}
+
+func (tb Builder) Build() (out struct {
+	x interface{}
+	s string
+}) {
+	out.x = nil
+	out.s = "hello!"
+	return
+}

test/fixedbugs/issue33866.dir/b.go

@@ -0,0 +1,15 @@
+// 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"
+
+type (
+	ABuilder = a.Builder
+)
+
+func Bfunc() ABuilder {
+	return ABuilder{}
+}

test/fixedbugs/issue33866.go

@@ -0,0 +1,9 @@
+// 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.
+
+// Issue 33866: assert in gccgo during compilation
+
+package ignored