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One C++ runtime per process, the graph and pack facts, bounded offlin… #1968

Job	Run time
examples: the curriculum builds	9m 19s
toolchain: musl + llvm	9m 33s
integration: mcpp builds & runs xlings	7m 16s
toolchain: gcc (cold self-host)	9m 37s
build + unit tests (linux x86_64, self-host)	17m 35s
	53m 20s