diagnostics: add support for nested diagnostics [PR116253]

Previously the diagnostic subsystem supported a one-deep
hierarchy via auto_diagnostic_group, for associating
notes with the warning/error they annotate; this only
affects SARIF output, not text output.

This patch adds support to the diagnostics subsystem for
capturing arbitrarily deep nesting structure within
diagnostic messages.

This patch:
* adds the ability to express nesting internally when
  building diagnostics
* captures the nesting in SARIF output in the form documented
  in SG15's P3358R0 ("SARIF for Structured Diagnostics") via
  a "nestingLevel" property
* adds a new experimental mode to text output to see the
  hierarchy, via:
  -fdiagnostics-set-output=text:experimental-nesting=yes
* adds test coverage via a plugin, which with the above
  option emits:
  • note: child 0
    • note: grandchild 0 0
    • note: grandchild 0 1
    • note: grandchild 0 2
  • note: child 1
    • note: grandchild 1 0
    • note: grandchild 1 1
    • note: grandchild 1 2
  • note: child 2
    • note: grandchild 2 0
    • note: grandchild 2 1
    • note: grandchild 2 2
  using '*' rather than '•' if the text_art::theme is ascii-only.

My hope is to eventually:
(a) use this to improve C++'s template diagnostics
(b) remove the "experimental" caveat from the the text output mode

but this patch doesn't touch the C++ frontend, leaving both of these
to followup work.

gcc/c-family/ChangeLog:
	PR other/116253
	* c-opts.cc (c_diagnostic_text_finalizer): Use
	text_output.build_indent_prefix for prefix to
	diagnostic_show_locus.

gcc/ChangeLog:
	PR other/116253
	* diagnostic-core.h (class auto_diagnostic_nesting_level): New.
	* diagnostic-format-sarif.cc (class sarif_builder): Update leading
	comment re nesting of diagnostics.
	(sarif_result::on_nested_diagnostic): Add nestingLevel property.
	* diagnostic-format-text.cc (on_report_diagnostic): If we're
	showing nested diagnostics, then print changes of location on a
	new line, indented, and update m_last_location.
	(diagnostic_text_output_format::build_prefix): If m_show_nesting,
	then potentially add indentation and a bullet point.
	(get_bullet_point_unichar): New.
	(use_unicode_p): New.
	(diagnostic_text_output_format::build_indent_prefix): New.
	* diagnostic-format-text.h
	(diagnostic_text_output_format::diagnostic_text_output_format):
	Initialize m_show_nesting and m_show_nesting_levels.
	(diagnostic_text_output_format::build_indent_prefix): New decl.
	(diagnostic_text_output_format::show_nesting_p): New accessor
	(diagnostic_text_output_format::show_locations_in_nesting_p):
	Likewise.
	(diagnostic_text_output_format::set_show_nesting): New.
	(diagnostic_text_output_format::set_show_locations_in_nesting):
	New.
	(diagnostic_text_output_format::set_show_nesting_levels): New.
	(diagnostic_text_output_format::m_show_nesting): New field.
	(diagnostic_text_output_format::m_show_locations_in_nesting): New
	field.
	(diagnostic_text_output_format::m_show_nesting_levels): New field.
	* diagnostic-global-context.cc
	(auto_diagnostic_nesting_level::auto_diagnostic_nesting_level):
	New.
	(auto_diagnostic_nesting_level::~auto_diagnostic_nesting_level):
	New.
	* diagnostic-show-locus.cc (layout_printer::print): Temporarily
	set DIAGNOSTICS_SHOW_PREFIX_EVERY_LINE.
	* diagnostic.cc (diagnostic_context::initialize): Update for
	renaming of m_nesting_depth to m_group_nesting_depth and
	initialize m_diagnostic_nesting_level.
	(diagnostic_context::finish): Update for renaming of
	m_nesting_depth to m_group_nesting_depth.
	(diagnostic_context::report_diagnostic): Likewise.
	(diagnostic_context::begin_group): Likewise.
	(diagnostic_context::end_group): Likewise.
	(diagnostic_context::push_nesting_level): New.
	(diagnostic_context::pop_nesting_level): New.
	(diagnostic_context::set_diagnostic_buffer): Update for renaming
	of m_nesting_depth to m_group_nesting_depth.  Assert that we don't
	have nested diagnostics.
	* diagnostic.h (diagnostic_context::push_nesting_level): New decl.
	(diagnostic_context::pop_nesting_level): New decl.
	(diagnostic_context::get_diagnostic_nesting_level): New accessor.
	(diagnostic_context::build_indent_prefix): New decl.
	(diagnostic_context::m_diagnostic_groups): Rename m_nesting_depth
	to m_group_nesting_depth and add field m_diagnostic_nesting_level.
	* doc/invoke.texi (fdiagnostics-add-output): Add note about
	"experimental" schemes, keys, and values.  Add keys
	"experimental-nesting", "experimental-nesting-show-locations",
	and "experimental-nesting-show-levels" to text scheme.
	* opts-diagnostic.cc (text_scheme_handler::make_sink): Add keys
	"experimental-nesting", "experimental-nesting-show-locations",
	and "experimental-nesting-show-levels".

gcc/testsuite/ChangeLog:
	PR other/116253
	* gcc.dg/plugin/diagnostic-test-nesting-sarif.c: New test.
	* gcc.dg/plugin/diagnostic-test-nesting-sarif.py: New test.
	* gcc.dg/plugin/diagnostic-test-nesting-text-indented-show-levels.c:
	New test.
	* gcc.dg/plugin/diagnostic-test-nesting-text-indented-unicode.c:
	New test.
	* gcc.dg/plugin/diagnostic-test-nesting-text-indented.c: New test.
	* gcc.dg/plugin/diagnostic-test-nesting-text-plain.c: New test.
	* gcc.dg/plugin/diagnostic_plugin_test_nesting.c: New test plugin.
	* gcc.dg/plugin/plugin.exp: Add the above.

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