tests/debuginfo/basic-stepping.rs: Explain why all lines are not steppable Some optimization passes [_improve_](https://github.com/rust-lang/compiler-team/issues/319) compile times. So we want to run some passes even with `-Copt-level=0`. That means that some debuggable lines can be optimized away. Document that as expected behavior. Closes https://github.com/rust-lang/rust/issues/33013. Replaces https://github.com/rust-lang/rust/pull/151426. See that PR for some discussion.