cmd/cgo: don't resolve typedefs in -godefs mode

In -godefs mode any typedefs that appear in struct fields and the like
will presumably be defined in the input file. If we resolve to the
base type, those cross-references will not work. So for -godefs mode,
keep the Go 1.10 behavior and don't resolve the typedefs in a loop.

Fixes #26644

Change-Id: I48cf72d9eb5016353c43074e6aff6495af326f35
Reviewed-on: https://go-review.googlesource.com/125995Reviewed-by: Keith Randall <khr@golang.org>

misc/cgo/testgodefs/fieldtypedef.go

+// Copyright 2018 The Go Authors. All rights reserve d.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// +build ignore
+
+package main
+
+/*
+struct S1 { int f1; };
+struct S2 { struct S1 s1; };
+typedef struct S1 S1Type;
+typedef struct S2 S2Type;
+*/
+import "C"
+
+type S1 C.S1Type
+type S2 C.S2Type

misc/cgo/testgodefs/test.bash

@@ -7,7 +7,7 @@
 # We are testing cgo -godefs, which translates Go files that use
 # import "C" into Go files with Go definitions of types defined in the
 # import "C" block.  Add more tests here.
-FILE_PREFIXES="anonunion issue8478"
+FILE_PREFIXES="anonunion issue8478 fieldtypedef"
 
 RM=
 for FP in $FILE_PREFIXES

src/cmd/cgo/gcc.go

@@ -180,6 +180,13 @@ func (p *Package) Translate(f *File) {
 		if len(needType) > 0 {
 			p.loadDWARF(f, needType)
 		}
+
+		// In godefs mode we're OK with the typedefs, which
+		// will presumably also be defined in the file, we
+		// don't want to resolve them to their base types.
+		if *godefs {
+			break
+		}
 	}
 	if p.rewriteCalls(f) {
 		// Add `import _cgo_unsafe "unsafe"` after the package statement.