cmd/cgo: unwrap typedef-chains before type checking

clang can emit some dwarf.VoidType which are wrapped by multiple
dwarf.TypedefType. We need to unwrap those before further processing.

Fixes #20129

Change-Id: I671ce6aef2dc7b55f1a02aec5f9789ac1b369643
Reviewed-on: https://go-review.googlesource.com/44772Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/cgo_test.go

@@ -79,5 +79,6 @@ func Test18126(t *testing.T)                 { test18126(t) }
 func Test20369(t *testing.T)                 { test20369(t) }
 func Test18720(t *testing.T)                 { test18720(t) }
 func Test20266(t *testing.T)                 { test20266(t) }
+func Test20129(t *testing.T)                 { test20129(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue20129.go

+// Copyright 2017 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 cgotest
+
+/*
+int issue20129 = 0;
+typedef void issue20129Void;
+issue20129Void issue20129Foo() {
+	issue20129 = 1;
+}
+typedef issue20129Void issue20129Void2;
+issue20129Void2 issue20129Bar() {
+	issue20129 = 2;
+}
+*/
+import "C"
+import "testing"
+
+func test20129(t *testing.T) {
+	if C.issue20129 != 0 {
+		t.Fatal("test is broken")
+	}
+	C.issue20129Foo()
+	if C.issue20129 != 1 {
+		t.Errorf("got %v but expected %v", C.issue20129, 1)
+	}
+	C.issue20129Bar()
+	if C.issue20129 != 2 {
+		t.Errorf("got %v but expected %v", C.issue20129, 2)
+	}
+}

src/cmd/cgo/gcc.go

@@ -2286,7 +2286,7 @@ func (c *typeConv) FuncType(dtype *dwarf.FuncType, pos token.Pos) *FuncType {
 	}
 	var r *Type
 	var gr []*ast.Field
-	if _, ok := dtype.ReturnType.(*dwarf.VoidType); ok {
+	if _, ok := base(dtype.ReturnType).(*dwarf.VoidType); ok {
 		gr = []*ast.Field{{Type: c.goVoid}}
 	} else if dtype.ReturnType != nil {
 		r = c.Type(unqual(dtype.ReturnType), pos)