GAS: Allow automatically assigned entries in the file table to be reassigned if the...

GAS: Allow automatically assigned entries in the file table to be reassigned if the source file specifically requests to use the assigned slot.

	PR 25878
	* dwarf2dbg.c (struct file_entry): Add auto_assigned field.
	(assign_file_to_slot): New function.  Fills in an entry in the
	files table.
	(allocate_filenum): Use new function.
	(allocate_filename_to_slot): Use new function.  If the specified
	slot entry is already in use, but was chosen automatically then
	reassign the automatic entry.