Fix CU: output of the --debug-dump=decodedline, the problem there
is that both file name and current directory is recorded, however
file name sometimes already contains absolute path. In which case
prefixing it with directory name results in an invalid pathname.

Only append directory name if the file name does not start with '/'.
This seems to DTRT.

Approved by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18290