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Isn't most of their code in Objective C? Does that use clang too?

clang does support Objective C (as does gcc).

Apple does look after a lot of plain old C (e.g. Core Foundation) and C++ (e.g. WebKit) too, so clang handles all three.

If I'm not mistaken, the drivers are also written in C++ (They used to be in Objective-C).