-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 intend to use it to replace the real source directory and real object
directory with constant values across all builds.

Reviewed by:	brooks
Differential Revision:	https://reviews.freebsd.org/D28632