Louis Dionne ed61d6a466 [libc++] Use the stdlib=<LIB> Lit feature instead of use_system_cxx_lib
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
2023-03-30 06:57:56 -04:00
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