Cleanup indexable tree streaming.

Cleanup indexable tree streaming.

gcc/

	* lto-section-out.c (lto_output_decl_index): Remove.
	(lto_output_field_decl_index): Move to lto-streamer-out.c
	(lto_output_fn_decl_index): Move to lto-streamer-out.c
	(lto_output_namespace_decl_index): Remove.
	(lto_output_var_decl_index): Remove.
	(lto_output_type_decl_index): Remove.
	(lto_output_type_ref_index): Remove.
	* lto-streamer-out.c (output_type_ref): Remove.
	(lto_get_index): New function.
	(lto_output_tree_ref): Remove.
	(lto_indexable_tree_ref): New function.
	(lto_output_var_decl_index): Move here from lto-section-out.c; simplify.
	(lto_output_fn_decl_index): Move here from lto-section-out.c; simplify.
	(stream_write_tree_ref): Update.
	(lto_output_tree): Update.
	* lto-streamer.h (lto_output_decl_index): Remove prototype.
	(lto_output_field_decl_index): Remove prototype.
	(lto_output_namespace_decl_index): Remove prototype.
	(lto_output_type_decl_index): Remove prototype.
	(lto_output_type_ref_index): Remove prototype.
	(lto_output_var_decl_index): Move.
	(lto_output_fn_decl_index): Move