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`
1d679446b0