go/gcimporter15: backport interface embedding support

Backports golang.org/cl/38392 from go/internal/gcimporter.

Updates golang/go#16369.

Change-Id: Ic805f96e6565590987a5dae9f0f76c206fceab05
Reviewed-on: https://go-review.googlesource.com/38429
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

go/gcimporter15/bimport.go

@@ -493,12 +493,13 @@ func (p *importer) typ(parent *types.Package) types.Type {
 			p.record(nil)
 		}
 
-		// no embedded interfaces with gc compiler
-		if p.int() != 0 {
-			errorf("unexpected embedded interface")
+		var embeddeds []*types.Named
+		for n := p.int(); n > 0; n-- {
+			p.pos()
+			embeddeds = append(embeddeds, p.typ(parent).(*types.Named))
 		}
 
-		t := types.NewInterface(p.methodList(parent), nil)
+		t := types.NewInterface(p.methodList(parent), embeddeds)
 		if p.trackAllTypes {
 			p.typList[n] = t
 		}

go/gcimporter15/gcimporter_test.go

@@ -249,7 +249,7 @@ var importedObjectTests = []struct {
 }{
 	{"math.Pi", "const Pi untyped float"},
 	{"io.Reader", "type Reader interface{Read(p []byte) (n int, err error)}"},
-	{"io.ReadWriter", "type ReadWriter interface{Read(p []byte) (n int, err error); Write(p []byte) (n int, err error)}"},
+	{"io.ReadWriter", "type ReadWriter interface{Reader; Writer}"},
 	{"math.Sin", "func Sin(x float64) float64"},
 	// TODO(gri) add more tests
 }