Latest clang is using openat during pre-processing.
Its using absolute paths anyway which somewhat defeats the point I'd have thought.
Anyway, patch below handles two simple cases of openat - where path is absolute
or fd is AT_FDCWD, both of which can be treated like normal open for purpose of logging.
If anyone has a clue how to handle the remaining case - ie find the pathname associated with the fd,
let me know...