* symtab.c (output_source_filename): Delete unnecessary forward decl.

	* symtab.c (output_source_filename): Delete unnecessary forward decl.
	(filename_seen_cache): New struct.
	(INITIAL_FILENAME_SEEN_CACHE_SIZE): New macro.
	(create_filename_seen_cache): New function.
	(clear_filename_seen_cache, delete_filename_seen_cache): New functions.
	(filename_seen): Delete arg "first".  New arg "cache".  All callers
	updated.
	(output_source_filename_data): New struct.
	(output_source_filename): Delete arg "first".  New arg "data".
	All callers updated.
	(sources_info): Delete local "first".  New locals "data", "cleanups".
	Rewrite to use filename_seen_cache.
	(add_partial_filename_data): Delete member "first".  New member
	"filename_seen_cache".  All uses updated.
	(make_source_files_completion_list): Rewrite to use
	filename_seen_cache.