Rollup merge of #153941 - Enselic:fully-steppable, r=saethlin

tests/debuginfo/basic-stepping.rs: Explain why all lines are not steppable

Some optimization passes [_improve_](rust-lang/compiler-team#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 rust-lang/rust#33013.

Replaces rust-lang/rust#151426. See that PR for some discussion.

Author: JonathanBrouwer