Merge r14289 from the BUF_REMOVAL branch to trunk.

Merge r14289 from the BUF_REMOVAL branch to trunk.
Change CLG_(get_debug_info) to not build up an absolute pathname in its
'file' parameter. Instead give it an additional parameter to hold the
directory name. Callers can then build up the absolute pathname if needed.
This change will come in handy soonish when VG_(get_filename_lineno) will be
changed and those buffers will disappear.
The change has a bit of ripple to get_fn_node_inseg and CLG_(get_file_node).

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14666