Function gather_pgo_profile 

pub(crate) fn gather_pgo_profile<'a>(
    sh: &'a Shell,
    ra_build_cmd: Cmd<'a>,
    target: &str,
    train_crate: PgoTrainingCrate,
) -> Result<PathBuf>

Decorates ra_build_cmd to add PGO instrumentation, and then runs the PGO instrumented Rust Analyzer on itself to gather a PGO profile.