`datum->syntax` in `report-syntax-error` should pass a `srcloc` for better error reporting

[benknoble]

qi/qi-lib/flow.rkt

Line 45 in ab3b9be

(report-syntax-error

[countvajhula]

We discussed in today's meeting that we can't pass a srcloc here because the this-syntax is already the synthesized deforested version and not the original user-written version, which isn't preserved through expansion. If michaelballantyne/syntax-spec#14 is addressed, that would be one way that would allow us to pass the original syntax here. But IIRC last we talked about that, we weren't able to identify this "original syntax" unambiguously (see You'll Miss Me When I'm Gone).