[libc++] Reformulate availability in terms of LLVM releases

To make it easier to maintain the availability macros for both upstream
developers and vendors, this patch reformulates availability macros
as a function of the upstream LLVM release that a feature was introduced
in. This way, upstream developers can easily use the appropriate LLVM
version, and vendors can simply fill in the platform version(s) in which
a LLVM version landed.

Author: ldionne