diagnostic-show-locus.c: rework handling of multiple labels

diagnostic-show-locus.c: rework handling of multiple labels

This patch improves the handling of large numbers of labels within a
rich_location: previously, overlapping labels could lead to an assertion
failure within layout::print_any_labels.  Also, the labels were printed
in reverse order of insertion into the rich_location.

This patch moves the determination of whether a vertical bar should
be printed for a line_label into the
  'Figure out how many "label lines" we need, and which
   one each label is printed in.'
step of layout::print_any_labels, rather than doing it as the lines
are printed.  It also flips the sort order, so that labels at the
same line/column are printed in order of insertion into the
rich_location.

I haven't run into these issues with our existing diagnostics, but it
affects a patch kit I'm working on that makes more extensive use of
labels.

gcc/ChangeLog:
	* diagnostic-show-locus.c (line_label::line_label): Initialize
	m_has_vbar.
	(line_label::comparator): Reverse the sort order by m_state_idx,
	so that when the list is walked backwards the labels appear in
	order of insertion into the rich_location.
	(line_label::m_has_vbar): New field.
	(layout::print_any_labels): When dealing with multiple labels at
	the same line and column, only print vertical bars for the one
	with the highest label_line.
	(selftest::test_one_liner_labels): Update test for multiple labels
	to expect the labels to be in the order of insertion into the
	rich_location.  Add a test for many such labels, where the column
	numbers are out-of-order relative to the insertion order.

From-SVN: r276371