[libc++] Support build failures when running LNT benchmarks (#185234)

It's rare but possible for the codebase not to build. When that happens,
we should carry on and still submit an empty LNT report for that order,
otherwise we'll get stuck thinking that order hasn't been benchmarked
yet.

GitOrigin-RevId: d7ff97db15026860f1f9c95e8fbf9f81e7120be3