gdb/dwarf: pass a file_entry to line_header::file_file_name

gdb/dwarf: pass a file_entry to line_header::file_file_name

In the following patch, there will be some callers of file_file_name
that will already have access to the file_entry object for which they
want the file name.  It would be inefficient to have them pass an index,
only for line_header::file_file_name to re-lookup the same file_entry
object.  Change line_header::file_file_name to accept a file_entry
object reference, instead of an index to look up.

I think this change makes sense in any case.  Callers that have an index
can first obtain a file_entry using line_header::file_name_at or
line_header::file_names.

When passing a file_entry object, we can assume that the file_entry's
index is valid, unlike when passing an index.  So, push the special case
about an invalid index to the sole current caller of file_file_name,
macro_start_file.  I think that error belongs there anyway, since it
specifically talks about "bad file number in macro information".

This requires recording the file index in the file_entry structure, so
add that.

Change-Id: Ic6e44c407539d92b7863d7ba82405ade17f384ad