Relax `ProtocolType` requirement on `ProtocolObject` #2276

Job	Run time
Check formatting	16s
Check MSRV	1m 33s
Lint macOS 64bit	1m 9s
Lint iOS 32bit	58s
Lint GNUStep + exceptions	51s
Lint header-translator	56s
Lint test-assembly	34s
Test macOS 12	2m 23s
Compiler UI	0s
Verify header translator output	0s
Check icrate features	0s
Test on iOS simulator w. dinghy	0s
Assembly tests	0s
Test Compiler-RT	0s
${{ matrix.name }}	0s
${{ matrix.name }}	0s
	8m 40s

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