Emit a warning if the doctest `main` function will not be run Fixes #140310. I think we could try to go much further like adding a "link" (ie UI annotations) on the `main` function in the doctest. However that will require some more computation, not sure if it's worth it or not. Can still be done in a follow-up if we want it. For now, this PR does two things: 1. Pass the `DiagCtxt` to the doctest parser to emit the warning. 2. Correctly generate the `Span` to where the doctest is starting (I hope the way I did it isn't too bad either...). cc `@fmease` r? `@notriddle`