Add fileprefixmap compiler feature

-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