[DSLX] emit warning: if the naming of channels appears reversed

A pending change will make it so that the typing of these channels is fully determined:

```
let (s, r) = chan;
```

Where `s` is the channel that supports the `send` family of operations, and `r` is the channel that supports the `recv` family of operations (all channels in XLS are half-duplex).

Sometimes people use `_p` and `_c` suffix to indicate "producer/consumer" or "_out/_in" to indicate "producer/consumer" -- seems like it'd be helpful to flag as a warning when the naming convention seems reversed from the actual directionality of the channel types.