diagnostics: support compact printing of secondary locations

diagnostics: support compact printing of secondary locations

gcc/ChangeLog:
	* diagnostic-show-locus.c: Include "gcc-rich-location.h".
	(layout::m_primary_loc): New field.
	(layout::layout): Initialize new field.  Move location filtering
	logic from here to...
	(layout::maybe_add_location_range): ...this new method.  Add
	support for filtering to just the lines already specified by other
	locations.
	(layout::will_show_line_p): New method.
	(selftest::test_add_location_if_nearby): New test function.
	(selftest::diagnostic_show_locus_c_tests): Call it.
	* gcc-rich-location.h (gcc_rich_location::add_location_if_nearby):
	New method.

From-SVN: r250133