* plugin.cc (class Plugin_rescan): Define new class.

	* plugin.cc (class Plugin_rescan): Define new class.
	(Plugin_manager::claim_file): Set any_claimed_.
	(Plugin_manager::save_archive): New function.
	(Plugin_manager::save_input_group): New function.
	(Plugin_manager::all_symbols_read): Create Plugin_rescan task if
	necessary.
	(Plugin_manager::new_undefined_symbol): New function.
	(Plugin_manager::rescan): New function.
	(Plugin_manager::rescannable_defines): New function.
	(Plugin_manager::add_input_file): Set any_added_.
	* plugin.h (class Plugin_manager): define new fields rescannable_,
	undefined_symbols_, any_claimed_, and any_added_.  Declare
	Plugin_rescan as friend.  Declare new functions.
	(Plugin_manager::Rescannable): Define type.
	(Plugin_manager::Rescannable_list): Define type.
	(Plugin_manager::Undefined_symbol_list): Define type.
	(Plugin_manager::Plugin_manager): Initialize new fields.
	* archive.cc (Archive::defines_symbol): New function.
	(Add_archive_symbols::run): Pass archive to plugins if any.
	* archive.h (class Archive): Declare defines_symbol.
	* readsyms.cc (Input_group::~Input_group): New function.
	(Finish_group::run): Pass input_group to plugins if any.
	* readsyms.h (class Input_group): Declare destructor.
	* symtab.cc (add_from_object): Pass undefined symbol to plugins if
	any.