Add source location directives in DSLX for trace/error logging

### What's hard to do? (limit 100 words)

In SystemVerilog (and similar constructs exist in various software languages), we can use `` `__FILE__`` and `` `__LINE__`` to access the current source location, which can be useful for traces and other logging.

The example given in LRM 22.13 is:

```
$display("Internal error: null handle at %s, line %d.",
`__FILE__, `__LINE__);
```

This would dump something like:

```
Internal error: null handle at platforms/deepsea/logic/gfc/rtl/foo.sv, line 7.
```

I don't think DSLX users currently have the ability to express this.

### Current best alternative workaround (limit 100 words)

As I don't think this can be expressed through DSLX currently, the source info will be omitted.

### Your view of the "best case XLS enhancement" (limit 100 words)

Provide similar directives in DSLX that users can use in `trace_fmt!` and possibly elsewhere to highlight where the dump is coming from.