rework cppgc for prototype chain #3812

Job	Run time
compute-jobs	2s
build-linux / linux coverage	3m 51s
build-linux / linux lint	56s
build-linux / linux lint-deps	1m 48s
build-linux / linux test	3m 36s
build-linux / linux test-ops	6m 6s
build-linux / linux test-publish	1m 46s
build-linux / linux test-miri	6m 53s
build-linux / linux test-valgrind	20s
build-linux-arm / linux-arm test	6m 2s
build-linux-arm / linux-arm test-miri	11m 26s
build-linux-arm / linux-arm lint	1m 47s
build-macos / macos test	3m 9s
build-macos / macos lint	1m 51s
build-windows / windows lint	2m 40s
build-windows / windows test	7m 3s
success	1s
	59m 17s

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