diagnostics: SARIF output: add "{start,end}TimeUtc" properties (§§3.20.7-8)

diagnostics: SARIF output: add "{start,end}TimeUtc" properties (§§3.20.7-8)

gcc/ChangeLog:
	* diagnostic-format-sarif.cc
	(make_date_time_string_for_current_time): New.
	(sarif_invocation::sarif_invocation): Set "startTimeUtc"
	property (§3.20.7).
	(sarif_invocation::prepare_to_flush): Set "endTimeUtc"
	property (§3.20.8).

gcc/testsuite/ChangeLog:
	* c-c++-common/diagnostic-format-sarif-file-1.c: Verify that we have
	"startTimeUtc" and "endTimeUtc" properties of the correct form.

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