Add --verify-only option to DWP.

Add --verify-only option to DWP.

gold/
	* dwp.cc (Dwo_file_entry): New type.
	(File_list): Use Dwo_file_entry.
	(Dwo_file::verify): New function.
	(Dwo_file::verify_dwo_list): New function.
	(Dwo_file::sized_verify_dwo_list): New function.
	(Dwo_name_info_reader::visit_compilation_unit): Add dwo_id to file
	list.
	(Dwp_options): New enum type.
	(dwp_options): Add --verify-only.
	(usage): Likewise.
	(main): Likewise.
	* dwp.h (gold_info): Add declaration.