gdb/dwarf: pass compilation directory to line header

gdb/dwarf: pass compilation directory to line header

The following patch changes line_header::file_file_name to prepend the
compilation directory to the file name, if needed.  For that, the line
header needs to know about the compilation directory.  Prepare for that
by adding a constructor that takes it as a parameter, and passing the
value down everywhere needed.  Add a second constructor for the special
case of building a line_header for doing a hash table lookup, since that
case doesn't require a compilation directory value.

Change-Id: Iba3ba0293e4e2d13a64b257cf9a3094684d54330