Add CFDate conversions to/from SystemTime #3310

Job	Run time
Check formatting	12s
Lint macOS x86_64	3m 22s
Check MSRV	55s
Lint iOS ARMv7s	3m 16s
Lint visionOS Aarch64 simulator	3m 23s
Lint GNUStep + exceptions	1m 24s
Lint header-translator	33s
Lint test-assembly	28s
Test macOS 15 Aarch64	1m 24s
Verify header translator output	1m 52s
Assembly tests	53s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	30s
Test GNUStep with libobjc2 v1.9	56s
Compiler UI	43s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.0	1m 9s
Test GNUStep with libobjc2 v2.1 on nightly	1m 17s
Test GNUStep 32bit	1m 12s
	23m 29s

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