[libc++] Feature-test macros should reflect configurations like _LIBCPP_HAS_NO_FILESYSTEM

[ldionne]

Currently, FTMs like __cpp_lib_filesystem take into account availability markup like _LIBCPP_AVAILABILITY_HAS_NO_FILESYSTEM_LIBRARY. In other words, if the deployment target doesn't support <filesystem> the FTM will reflect that. That's good.

However, FTMs don't reflect carve-outs like _LIBCPP_HAS_NO_FILESYSTEM. In fact, generated tests like libcxx/test/std/language.support/support.limits/support.limits.general/filesystem.version.compile.pass.cpp are disabled with // UNSUPPORTED: no-filesystem, which means that we basically don't test the presence or absence of these FTMs in the case of carved-out configurations.

Instead, FTMs should reflect that the library has been configured without support for X or Y.

[ldionne]

I think this was fixed by #80168.