Use Clang's `-fmodules` #3180

Job	Run time
Check formatting	11s
Lint macOS x86_64	2m 46s
Check MSRV	53s
Lint iOS ARMv7s	2m 55s
Lint visionOS Aarch64 simulator	2m 48s
Lint GNUStep + exceptions	1m 27s
Lint header-translator	36s
Lint test-assembly	38s
Test macOS 15 Aarch64	2m 6s
Assembly tests	52s
Verify header translator output	1m 26s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	22s
Test GNUStep with libobjc2 v1.9	1m 11s
Test GNUStep with libobjc2 v2.0	1m 42s
Compiler UI	58s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	1m 9s
Test GNUStep 32bit	1m 20s
	23m 20s

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