Cleanup global decl stream reference streaming, part 2

Cleanup global decl stream reference streaming, part 2

gcc/ChangeLog:

2020-06-01  Jan Hubicka  <hubicka@ucw.cz>

	* lto-streamer.h (enum LTO_tags): Remove LTO_field_decl_ref,
	LTO_function_decl_ref, LTO_label_decl_ref, LTO_namespace_decl_ref,
	LTO_result_decl_ref, LTO_type_decl_ref, LTO_type_ref,
	LTO_const_decl_ref, LTO_imported_decl_ref,
	LTO_translation_unit_decl_ref, LTO_global_decl_ref and
	LTO_namelist_decl_ref; add LTO_global_stream_ref.
	* lto-streamer-in.c (lto_input_tree_ref): Simplify.
	(lto_input_scc): Update.
	(lto_input_tree_1): Update.
	* lto-streamer-out.c (lto_indexable_tree_ref): Simlify.
	* lto-streamer.c (lto_tag_name): Update.