[libc++] Fix backdeployment annotations for std::filesystem

In 1fa27f2a, we made <filesystem>'s iterator types model concepts
from <ranges>, but we forgot to add the appropriate availability
annotations. This broke back-deployment to platforms that don't have
<filesystem> for which we have availability annotations.

For some reason, this wasn't caught by our back-deployment CI.
I believe this is due to the fact that we use a slightly older
compiler in the CI, and perhaps that compiler does not honour
our `#pragma clang attribute push` properly.

Differential Revision: https://reviews.llvm.org/D114456