[libc++] Sort `_LIBCPP_INTRODUCED_IN_LLVM_<X>` usages in reverse chronological order (#176857)

I noticed that the various `_LIBCPP_INTRODUCED_IN_LLVM_<ver>` usages in
`availability.h` were defined a bit all over the place. I think it'd
make the most sense to sort them in reverse chronological order (like
their definitions).

GitOrigin-RevId: 783fd2f9d583850ecacdf93cd65f903b64d0cf4d