sarif output: introduce sarif_serialization_format

sarif output: introduce sarif_serialization_format

The SARIF 2.1.0 spec says that although a "SARIF log file SHALL contain
a serialization of the SARIF object model into the JSON format ... in the
future, other serializations might be defined." (§3.1)

I've been experimenting with alternative serializations of SARIF (CBOR
and JSON5 for now).  To help with these experiments, this patch adds a
new param "serialization" to -fdiagnostics-add-output='s "sarif" scheme.

For now this must have value "json", but will be helpful for any
followup patches.

gcc/ChangeLog:
	* diagnostic-format-sarif.cc
	(sarif_serialization_format_json::write_to_file): New.
	(sarif_builder::m_formatted): Replace field with...
	(sarif_builder::m_serialization_format): ...this.
	(sarif_builder::sarif_builder): Update for field change.
	(sarif_builder::flush_to_file): Call m_serialization_format's
	write_to_file vfunc.
	(sarif_output_format::sarif_output_format): Replace param
	"formatted" with "serialization_format".
	(sarif_stream_output_format::sarif_output_format): Likewise.
	(sarif_file_output_format::sarif_file_output_format): Likewise.
	(diagnostic_output_format_init_sarif_stderr): Make a
	sarif_serialization_format_json and pass it to
	diagnostic_output_format_init_sarif.
	(diagnostic_output_format_open_sarif_file): Split out into...
	(diagnostic_output_file::try_to_open): ...this, adding
	"serialization_kind" param.
	(diagnostic_output_format_init_sarif_file): Update for new param
	to diagnostic_output_format_open_sarif_file.  Make a
	sarif_serialization_format_json and pass it to
	diagnostic_output_format_init_sarif.
	(diagnostic_output_format_init_sarif_stream): Make a
	sarif_serialization_format_json and pass it to
	diagnostic_output_format_init_sarif.
	(make_sarif_sink): Replace param "formatted" with "serialization".
	(selftest::test_make_location_object): Update for changes to
	sarif_builder ctor.
	* diagnostic-format-sarif.h (enum class sarif_serialization): New.
	(diagnostic_output_format_open_sarif_file): Add param
	"serialization_kind".
	(class sarif_serialization_format): New.
	(class sarif_serialization_format_json): New.
	(make_sarif_sink): Replace param "formatted" with
	"serialization_format".
	* diagnostic-output-file.h (diagnostic_output_file::try_to_open):
	New decl.
	* diagnostic.h (enum diagnostics_output_format): Tweak comments.
	* doc/invoke.texi (-fdiagnostics-add-output): Add "serialization"
	param to sarif scheme.
	* libgdiagnostics.cc (sarif_sink::sarif_sink): Update for change
	to make_sarif_sink.
	* opts-diagnostic.cc (sarif_scheme_handler::make_sink): Add
	"serialization" param and pass it on to make_sarif_sink.

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