By adding a couple of members to Link, we are
now able to trace the target function name and
the file:line where the Link instance in question
was created (provided the LINK macro is used).

This gives the invaluable ability to track down the
source of a Link instance in the debugger, provided
we have enabled this feature, which is enabled in
DBG_UTIL automatically, unless explicitly enabled.

Of course it is also possible to judiciously add
LOG/fprintf statements to chase this info, if not
outright track all links, if so we wish, by dumping
from Link::Call, or at construction time of Link.

Change-Id: Iab1dce31a179d28aaa1f20228e9e0405973b5e9b
Reviewed-on: https://gerrit.libreoffice.org/73478
Tested-by: Jenkins
Reviewed-by: Jan Holesovsky <kendy@collabora.com>