red_knot_python_semantic: remove `InferContext::report_diagnostic`
... and replace it with use of `report()`.
Interestingly, this is the only instance of `report_diagnostic` used
directly, and thus anticipated to be the only instance of using
`report()`. If this ends up being a true single use method, we could
make it less generic and tailored specifically to "reveal type."
Two other things to note:
I left the "primary message" as empty. This avoids changing snapshots.
I address this in a subsequent commit.
The creation of a diagnostic here is a bit verbose/annoying. Certainly
more so than it was. This is somewhat expected since our diagnostic
model is more expressive and because we don't have a proc macro. I
avoided creating helpers for this case since there's only one use of
`report()`. But I expect to create helpers for the `lint()` case.
