Make ClassType private to implement #3059

Job	Run time
Check MSRV	52s
Lint macOS 64bit	2m 37s
Check formatting	7s
Lint iOS 32bit	2m 13s
Lint GNUStep + exceptions	1m 24s
Lint header-translator	34s
Lint test-assembly	32s
Test macOS 12	4m 30s
Verify header translator output	1m 0s
Assembly tests	52s
${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	20s
Test GNUStep with libobjc2 v1.9	1m 5s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.0	1m 21s
Compiler UI	1m 47s
Test GNUStep with libobjc2 v2.1 on nightly	1m 32s
Test GNUStep 32bit	1m 0s
	21m 46s

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