go/doc: set receiver type position for embedded methods

This was a bug that didn't manifest itself before CL 7674044;
but with that CL and without this fix, the go/doc tests fail.
(The bug fixed by 7674044 and the bug fixed here cancelled
each other out w/ respect to the go/doc tests).

R=rsc
CC=golang-dev
https://golang.org/cl/7628045

src/pkg/go/doc/reader.go

@@ -533,10 +533,13 @@ func customizeRecv(f *Func, recvTypeName string, embeddedIsPtr bool, level int)
 
 	// copy existing receiver field and set new type
 	newField := *f.Decl.Recv.List[0]
+	origPos := newField.Type.Pos()
 	_, origRecvIsPtr := newField.Type.(*ast.StarExpr)
-	var typ ast.Expr = ast.NewIdent(recvTypeName)
+	newIdent := &ast.Ident{NamePos: origPos, Name: recvTypeName}
+	var typ ast.Expr = newIdent
 	if !embeddedIsPtr && origRecvIsPtr {
-		typ = &ast.StarExpr{X: typ}
+		newIdent.NamePos++ // '*' is one character
+		typ = &ast.StarExpr{Star: origPos, X: newIdent}
 	}
 	newField.Type = typ