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mcpp emit build-database, program probes as argument vectors, and xli… #1957

Job	Run time
build + unit tests (linux x86_64, self-host)	14m 13s
integration: mcpp builds & runs xlings	9m 52s
toolchain: musl + llvm	11m 21s
toolchain: gcc (cold self-host)	11m 40s
examples: the curriculum builds	11m 22s
	58m 28s