Cannot parse a herestring that follows a file redirect

The grammar, as of d1a1a3fe7189fda, cannot parse a herestring that follows a file redirect:

```bash
rev > output <<< hello
```

(This will write `olleh` to a file named `output`, FWIW.)

The grammar can parse the equivalent statement, when the file redirect and herestring are switched:

```bash
rev <<< hello > output
```

Possibly related: #260