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The use_system_cxx_lib Lit feature was only used for back-deployment testing. However, one immense hole in that setup was that we didn't have a proper way to test Apple's own libc++ outside of back-deployment, which was embodied by the fact that we needed to define _LIBCPP_DISABLE_AVAILABILITY when testing (see change in libcxx/utils/libcxx/test/params.py). This led to the apple-system testing configuration not checking for availability markup, which is obviously quite bad since the library we ship actually has availability markup. Using stdlib=<VENDOR>-libc++ instead to encode back-deployment restrictions on tests is simpler and it makes it possible to naturally support tests such as availability markup checking even in the tip-of-trunk Apple-libc++ configuration. Differential Revision: https://reviews.llvm.org/D146366