[libc++] Mark <filesystem> as unavailable on Apple platforms using pragmas
Also add the corresponding XFAILs to tests that require filesystem.
The approach taken to mark <filesystem> as unavailable in this patch
is to mark all the header as unavailable using #pragma clang attribute.
Marking each declaration using the attribute is more intrusive and
does not provide a lot of value right now because pretty much everything
in <filesystem> requires dylib support, often transitively.
This is an alternative to https://reviews.llvm.org/D59093.
A similar (but partial) patch was already applied in r356558.
Reviewers: mclow.lists, EricWF, serge-sans-paille
Subscribers: christof, jkorous, dexonsmith, libcxx-commits
Differential Revision: https://reviews.llvm.org/D59224