The `_LIBCPP_DISABLE_AVAILABILITY` macro was removed in afae1a5f32bb as an
intended no-op. It turns out that some projects are making use of that
macro to work around a Clang bug with availability annotations that
still exists: https://github.com/llvm/llvm-project/issues/134151.
Since that Clang bug still hasn't been fixed, I feel that we must sill
honor that unfortunate macro until we've figured out how to get rid of
it without breaking code.
(cherry picked from commit 25fc52e655fb4bfd3bb89948d5cbfe011e1b8984)
be087ab359