tools/cmd/stringer: fake import C.

Avoid error "could not import C (can't find import: C)"
Fixes golang/go#9169.

LGTM=adonovan, r
R=golang-codereviews, adonovan, r
CC=golang-codereviews
https://golang.org/cl/184730043

cmd/stringer/stringer.go

@@ -261,7 +261,7 @@ func (g *Generator) parsePackage(directory string, names []string, text interfac
 // check type-checks the package. The package must be OK to proceed.
 func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) {
 	pkg.defs = make(map[*ast.Ident]types.Object)
-	var config types.Config
+	config := types.Config{FakeImportC: true}
 	info := &types.Info{
 		Defs: pkg.defs,
 	}

cmd/stringer/testdata/cgo.go

@@ -0,0 +1,32 @@
+// 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.
+
+// Import "C" shouldn't be imported.
+
+package main
+
+/*
+#define HELLO 1
+*/
+import "C"
+
+import "fmt"
+
+type Cgo uint32
+
+const (
+	// MustScanSubDirs indicates that events were coalesced hierarchically.
+	MustScanSubDirs Cgo = 1 << iota
+)
+
+func main() {
+	_ = C.HELLO
+	ck(MustScanSubDirs, "MustScanSubDirs")
+}
+
+func ck(day Cgo, str string) {
+	if fmt.Sprint(day) != str {
+		panic("cgo.go: " + str)
+	}
+}