This makes toolchain independent of the path it was built in by
rewriting all absolute paths embedded in sources and debug info
into relative ones.
@leonardchan sorry for the inconvenience, but Phabricator has a bug/missing feature where it doesn't send mail for bug updates that contain no user-authored text, so approving a patch without writing anything in the text box doesn't send an email to the mailing lrist (the mailing list is still considered the canonical source of truth for reviews, etc) and thus makes it looks like patches are being committed without review. When approving a patch through phab, please write something (eg: "Thanks!", "Looks good!", etc) in the text box to workaround this bug.