diagnostics: SARIF output: add sarif_object subclasses throughout

diagnostics: SARIF output: add sarif_object subclasses throughout

No functional change intended.

gcc/ChangeLog:
	* diagnostic-format-sarif.cc: Introduce subclasses of sarif_object
	for all aspects of the spec that we're using.  Replace almost all
	usage of json::object with uses of these subclasses, the only
	remaining use of json::object being for originalUriBaseIds, as per
	SARIF 2.1.0 §3.14.14.  This stronger typing makes it considerably
	easier to maintain validity against the schema.
	* diagnostic-format-sarif.h (class sarif_logical_location): New.
	(make_sarif_logical_location_object): Convert return type from
	json::object * to sarif_logical_location *.

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