Avoid streaming stray references.

this patch avoids stremaing completely useless stray references to gobal decl
stream.  I am re-testing the patch (rebased to current tree) on x86_64-linux
and intend to commit once testing finishes.

gcc/ChangeLog:

2020-05-22  Jan Hubicka  <hubicka@ucw.cz>

	* lto-streamer-out.c (lto_output_tree): Do not stream final ref if
	it is not needed.

gcc/lto/ChangeLog:

2020-05-22  Jan Hubicka  <hubicka@ucw.cz>

	* lto-common.c (lto_read_decls): Do not skip stray refs.