Implement lint for obligations broken by never type fallback change
This is the second (and probably last major?) lint required for the never type fallback change.
The idea is to check if the code errors with `fallback = ()` and if it errors with `fallback = !` and if it went from "ok" to "error", lint.
I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that (cc `@jackh726,` iirc you had some ideas?)
r? `@compiler-errors`
Thanks `@BoxyUwU` with helping with trait solver stuff when I was implementing the initial version of this lint.
Tracking:
- https://github.com/rust-lang/rust/issues/123748
b6e5e3ffbb