diagnostics: implement highlight-a vs -b in HTML output [PR116792]

diagnostics: implement highlight-a vs -b in HTML output [PR116792]

Update HTML output so that it renders highlight-a vs highlight-b
via <span> tags in the message itself, in the quoted source line,
in the underlines, and in the labels and their vertical bars.

Example output can be seen at:
  https://dmalcolm.fedorapeople.org/gcc/2025-05-28/diagnostic-ranges.c.html

gcc/ChangeLog:
	PR other/116792
	* diagnostic-format-html.cc (HTML_STYLE): Add ".highlight-a" and
	".highlight-b".
	(html_builder::make_element_for_diagnostic): Handle begin_color
	and end_color.
	* diagnostic-show-locus.cc (to_html::to_html): Add "richloc"
	param and use it to initialize m_richloc.
	(to_html::colorize_text_for_range_idx): Drop.
	(to_html::get_location_range_by_idx): New.
	(to_html::get_highlight_color_for_range_idx): New.
	(to_html::m_richloc): New field.
	(print_html_span_start): Update for new param of to_html ctor.
	(line_printer::m_was_in_range_p): New field.
	(line_printer::m_last_range_idx): New field.
	(layout_printer<Sink>::print_source_line): Use set_in_range
	and set_outside_range rather than colorization calls.
	(layout_printer<Sink>::set_in_range): New.
	(layout_printer<Sink>::set_outside_range): New.
	(layout_printer<Sink>::print_annotation_line): Use set_in_range
	and set_outside_range rather than colorization calls.
	(layout_printer<to_text>::begin_label): Convert param from label
	to state_idx.  Add "is_label_text" param and use it to guard logic
	for turning off colorization within paths.
	(layout_printer<to_html>::begin_label): Likewise.  Push <span>
	for any highlight color.
	(layout_printer<to_text>::end_label): Likewise.
	(layout_printer<to_text>::end_label): Likewise, popping the
	<span>.
	(layout_printer<Sink>::print_any_labels): Convert begin/end_label
	calls to pass in state_idx rather than label.  Use begin/end_label
	rather than colorization calls.
	(layout_printer<Sink>::layout_printer): Likewise.
	(layout_printer<Sink>::layout_printer): Initialize new fields.
	(diagnostic_source_print_policy::print_as_html): Update for new
	param of to_html ctor.

gcc/testsuite/ChangeLog:
	PR other/116792
	* gcc.dg/format/diagnostic-ranges-html.py: New test script.
	* gcc.dg/format/diagnostic-ranges.c: Add HTML generation to
	options, and invoke the new script to check the HTML output.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>