RFC: Adding availability attributes to the libc++ headers

Manman and I are working on adding availability attributes to the libc++
headers to express to `clang` which dylib feautures are available on
Apple platforms (and at what versions).

I'd love some feedback! Please read on.

What are availability attributes?

Sadly forgot the attachment. Got it this time.

WIP-libcxx-availability.patch (13.2 KB)