Handle all modules being serialized during LTO #2110

Job	Run time
build (--mini-tests)	46s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	59s
build (--std-tests)	52s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	48s
build (--test-libcore)	52s
success_gcc12	4s
	4m 21s

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