cmd/cgo: for typedef of untagged struct, use typedef name in C code

Fixes #8148.

LGTM=cookieo9, rsc
R=rsc, cookieo9
CC=golang-codereviews
https://golang.org/cl/103080043

misc/cgo/test/issue8148.go

+// Copyright 2014 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 8148.  A typedef of an unnamed struct didn't work when used
+// with an exported Go function.  No runtime test; just make sure it
+// compiles.
+
+package cgotest
+
+/*
+typedef struct { int i; } T;
+
+int issue8148Callback(T*);
+
+static int get() {
+	T t;
+	t.i = 42;
+	return issue8148Callback(&t);
+}
+*/
+import "C"
+
+//export issue8148Callback
+func issue8148Callback(t *C.T) C.int {
+	return t.i
+}
+
+func Issue8148() int {
+	return int(C.get())
+}

src/cmd/cgo/gcc.go

@@ -1283,6 +1283,11 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		if isStructUnionClass(sub.Go) || *godefs || *cdefs {
 			t.Go = sub.Go
 
+			if isStructUnionClass(sub.Go) {
+				// Use the typedef name for C code.
+				typedef[sub.Go.(*ast.Ident).Name].C = t.C
+			}
+
 			// If we've seen this typedef before, and it
 			// was an anonymous struct/union/class before
 			// too, use the old definition.