https://gcc.gnu.org/ml/gcc-patches/2019-10/msg01080.html
looking up a line map takes a non-constant line_maps object, which is confusing.
This makes the caching fields mutable, so permits a constant object, as one might expect for a lookup.

	* include/line-map.h (struct maps_info_ordinary): Make cache
	mutable.
	(struct maps_info_macro): Likewise.
	(LINEMAPS_CACHE): Remove non-ref accessor. Constify ref accessor.
	(LINEMAPS_ORDINARY_CACHE, LINEMAPS_MACRO_CACHE): Likewise.
	(LINEMAPS_ORDINARY_MAP_AT, LINEMAPS_MACRO_MAP_AT): Use
	LINEMAPS_USED and LINEMAPS_MAP_AT.
	(linemap_lookup): Constify line_map arg.
	linemap.c (linemap_ordinary_map_lookup, linemap_macro_map_lookup):
	Constify line_map arg.

From-SVN: r276994