re PR lto/67548 (LTO drops weak binding with "ld -r")

	PR lto/67548
	* lto-plugin.c (linker_output, linker_output_set): New statics.
	(all_symbols_read_handler): Add -flinker-output option.
	(onload): Record linker_output info.

	* ipa-visibility.c (cgraph_externally_visible_p,
	varpool_node::externally_visible_p): When doing incremental linking,
	hidden symbols may be still used later.
	(update_visibility_by_resolution_info): Do not drop weak during
	incremental link.
	(function_and_variable_visibility): Fix formating.
	* flag-types.h (lto_linker_output): Declare.
	* common.opt 9flag_incremental_link): New flag.

	* lto-lang.c (lto_post_options): Process flag_lto_linker_output.
	* lang.opt (lto_linker_output): New enum.
	(flinker_output): New flag.

From-SVN: r230915