macOS 15 fails line-tables-only because of change in C debuginfo

~~cc @madsmtm If you recognize this off the top of your head: https://github.com/rust-lang/backtrace-rs/actions/runs/17957211691/job/51071470672~~ A regression has been surfaced by a recent change in the runner images from macOS 14 to macOS 15. The effect of the regression is that when we compile some C code using `-g1`, which typically adds debuginfo, using an automatically-determined C toolchain by the logic in the `cc` crate, we no longer have frames for the C code in our backtrace. The root of the regression might be in backtrace-rs, it might be in cc-rs, and it might be in the Xcode toolchain packaged by https://github.com/actions/runner-images. This might be because of an Apple-specific change, or it might be because of a change in upstream Clang. Currently nothing has been eliminated from consideration.