libcc1: use "override"

This changes libcc1 to use "override" where appropriate.

libcc1

	* libcp1.cc (class compiler_triplet_regexp)
	(class compiler_driver_filename, class libcp1_connection): Use
	"override".
	* libcc1.cc (class compiler_triplet_regexp)
	(class compiler_driver_filename, class libcc1_connection): Use
	"override".