diagnostics: SARIF output: tweak output for UNKNOWN_LOCATION

diagnostics: SARIF output: tweak output for UNKNOWN_LOCATION

gcc/ChangeLog:
	* diagnostic-format-sarif.cc (sarif_builder::make_locations_arr):
	Don't add entirely empty location objects, such as for
	UNKNOWN_LOCATION.
	(test_sarif_diagnostic_context::test_sarif_diagnostic_context):
	Add param "main_input_filename".
	(selftest::test_simple_log): Provide above param.  Verify that
	"locations" is empty.
	(selftest::test_simple_log_2): New.
	(selftest::diagnostic_format_sarif_cc_tests): Call it.

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