This was added inconsistently in
19fd9039ca242f408493b5c662f9d908eab8555e; Windows doesn't have the
aligned_alloc function (neither MSVC nor MinGW toolchains) and we don't
define _LIBCPP_HAS_ALIGNED_ALLOC while building libcxx.
Differential Revision: https://reviews.llvm.org/D103399