pack fields in line-map data structures

As pointed out by pahole.

2022-01-18  Richard Biener  <rguenther@suse.de>

libcpp/
	* include/line-map.h (class line_maps): Re-arrange fields
	to minimize padding.
	(class rich_location): Likewise.
	* line-map.cc (rich_location::rich_location): Adjust.