rework cppgc for prototype chain #3809

Job	Run time
compute-jobs	4s
build-linux / linux test	3m 1s
build-linux / linux lint	1m 6s
build-linux / linux lint-deps	1m 51s
build-linux / linux coverage	3m 3s
build-linux / linux test-miri	3m 2s
build-linux / linux test-ops	3m 1s
build-linux / linux test-publish	1m 34s
build-linux / linux test-valgrind	20s
build-linux-arm / linux-arm test	3m 3s
build-linux-arm / linux-arm test-miri	1m 55s
build-linux-arm / linux-arm lint	1m 47s
build-macos / macos lint	2m 27s
build-macos / macos test	2m 44s
build-windows / windows lint	2m 48s
build-windows / windows test	2m 54s
success	1s
	34m 41s

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