diagnostics: make experimental-html sink prettier [PR116792]

This patch to the "experimental-html" diagnostic sink:
* adds use of the PatternFly 3 CSS library (via an optional link
  in the generated html to a copy in a CDN)
* uses PatternFly's "alert" pattern to show severities for diagnostics,
  properly nesting "note" diagnostics for diagnostic groups.
  Example:
    before: https://dmalcolm.fedorapeople.org/gcc/2025-06-10/before/diagnostic-ranges.c.html
     after: https://dmalcolm.fedorapeople.org/gcc/2025-06-10/after/diagnostic-ranges.c.html

* adds initial support for logical locations and physical locations
* adds initial support for multi-level nested diagnostics such as those
  for C++ concepts diagnostics.  Ideally this would show a clickable
  disclosure widget to expand/collapse a level, but for now it uses
  nested <ul> elements with <li> for the child diagnostics.
  Example:
    before: https://dmalcolm.fedorapeople.org/gcc/2025-06-10/before/nested-diagnostics-1.C.html
     after: https://dmalcolm.fedorapeople.org/gcc/2025-06-10/after/nested-diagnostics-1.C.html

gcc/ChangeLog:
	PR other/116792
	* diagnostic-format-html.cc: Include "diagnostic-path.h" and
	"diagnostic-client-data-hooks.h".
	(html_builder::m_logical_loc_mgr): New field.
	(html_builder::m_cur_nesting_levels): New field.
	(html_builder::m_last_logical_location): New field.
	(html_builder::m_last_location): New field.
	(html_builder::m_last_expanded_location): New field.
	(HTML_STYLE): Add "white-space: pre;" to .source and .annotation.
	Add "gcc-quoted-text" CSS class.
	(html_builder::html_builder): Initialize the new fields.  If CSS
	is enabled, add CDN links to PatternFly 3 stylesheets.
	(html_builder::add_stylesheet): New.
	(html_builder::on_report_diagnostic): Add "alert" param to
	make_element_for_diagnostic, setting it by default, but unsetting
	it for nested diagnostics below the top level.  Use
	add_at_nesting_level for nested diagnostics.
	(add_nesting_level_attr): New.
	(html_builder::add_at_nesting_level): New.
	(get_pf_class_for_alert_div): New.
	(get_pf_class_for_alert_icon): New.
	(get_label_for_logical_location_kind): New.
	(add_labelled_value): New.
	(html_builder::make_element_for_diagnostic): Add leading comment.
	Add "alert" param.  Drop class="gcc-diagnostic" from <div> tag,
	instead adding the class for a PatternFly 3 alert if "alert" is
	true, and adding a <span> with an alert icon, both according to
	the diagnostic severity.  Add a severity prefix to the message for
	alerts.  Add any metadata/option text as suffixes to the message.
	Show any logical location.  Show any physical location.  Don't
	show the locus if the last location is unchanged within the
	diagnostic_group.  Wrap any execution path element in a
	<div id="execution-path"> and add a label to it.  Wrap any
	generated patch in a <div id="suggested-fix"> and add a label
	to it.
	(selftest::test_simple_log): Update expected HTML.

gcc/testsuite/ChangeLog:
	PR other/116792
	* gcc.dg/html-output/missing-semicolon.py: Update for changes
	to diagnostic elements.
	* gcc.dg/format/diagnostic-ranges-html.py: Likewise.
	* gcc.dg/plugin/diagnostic-test-metadata-html.py: Likewise.  Drop
	out-of-date comment.
	* gcc.dg/plugin/diagnostic-test-paths-2.py: Likewise.
	* gcc.dg/plugin/diagnostic-test-paths-4.py: Likewise.  Drop
	out-of-date comment.
	* gcc.dg/plugin/diagnostic-test-show-locus.py: Likewise.
	* lib/htmltest.py (get_diag_by_index): Update to use search by id.
	(get_message_within_diag): Update to use search by class.

libcpp/ChangeLog:
	PR other/116792
	* include/line-map.h (typedef expanded_location): Convert to...
	(struct expanded_location): ...this.
	(operator==): New decl, for expanded_location.
	(operator!=): Likewise.
	* line-map.cc (operator==): New decl, for expanded_location.

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

libcpp/include/line-map.h

@@ -1280,7 +1280,7 @@ linemap_location_before_p (const line_maps *set,
   return linemap_compare_locations (set, loc_a, loc_b) >= 0;
 }
 
-typedef struct
+struct expanded_location
 {
   /* The name of the source file involved.  */
   const char *file;
@@ -1294,7 +1294,18 @@ typedef struct
 
   /* In a system header?. */
   bool sysp;
-} expanded_location;
+};
+
+extern bool
+operator== (const expanded_location &a,
+	    const expanded_location &b);
+inline bool
+operator!= (const expanded_location &a,
+	    const expanded_location &b)
+{
+  return !(a == b);
+}
+
 
 /* This is enum is used by the function linemap_resolve_location
    below.  The meaning of the values is explained in the comment of

libcpp/line-map.cc

@@ -1949,6 +1949,28 @@ linemap_expand_location (const line_maps *set,
   return xloc;
 }
 
+bool
+operator== (const expanded_location &a,
+	    const expanded_location &b)
+{
+  /* "file" can be null; for them to be equal they must both
+     have either null or nonnull values, and if non-null
+     they must compare as equal.  */
+  if ((a.file == nullptr) != (b.file == nullptr))
+    return false;
+  if (a.file && strcmp (a.file, b.file))
+    return false;
+
+  if (a.line != b.line)
+    return false;
+  if (a.column != b.column)
+    return false;
+  if (a.data != b.data)
+    return false;
+  if (a.sysp != b.sysp)
+    return false;
+  return true;
+}
 
 /* Dump line map at index IX in line table SET to STREAM.  If STREAM
    is NULL, use stderr.  IS_MACRO is true if the caller wants to