* lto-streamer-out.c (hash_string_slot_node): Hash string based on its
	length.
	(string_slot_free): Remove
	(create_output_block): Initialize obstack.
	(destroy_output_block): Free obstack.
	(lto_string_index): Add PERSISTENT parameter; do not duplicate
	the string unless it needs to be added into the hash.
	(lto_output_string_with_length): Add persistent attribute;
	handle NULL strings.
	(lto_output_string): Add PERSISTENT parameter.
	(output_string_cst, output_identifier): Simplify.
	(lto_output_location_bitpack): Update.
	(lto_output_builtin_tree): Update.
	* lto-streamer.h (struct output_block): Add obstack.
	(lto_output_string, lto_output_string_with_length): Remove declarations;
	functions are static now.

From-SVN: r174397