-ffile-prefix-map=<old>=<new> is a compiler feature first added in
GCC 8, and implemented for clang 10. It remaps old paths to new paths
in both debug information and FILE and BASE_FILE macros. It can
be used to improve reproducibility or to hide local system directories.
I plan to use it in future reviews to replace the real source directory
and real object directory with constant values across all builds.