Latest clang uses openat(2).

If the pathname is absolute or dirfd is AT_FDCWD we can
handle it exactly like open(2).
Otherwise we output an A record to indicate that the path of
an open directory needs to be used (earlier in the trace).

Also filemon_pid_check needs to hold proctree_lock
and use proc_realparent()

Differential Revision: D2810
Reviewed by: jhb
Approved by: re