Function copy_cgu_workproduct_to_incr_comp_cache_dir 

pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
    sess: &Session,
    cgu_name: &str,
    files: &[(&'static str, &Path)],
    known_links: &[PathBuf],
) -> (WorkProductId, WorkProduct)

Copies a CGU work product to the incremental compilation directory, so next compilation can find and reuse it.

Panics when incr comp is disabled.