Still expose std::align_val_t in C++17 even if we don't have aligned new/delete.

r292564 disabled the aligned new/delete overloads on platforms without
posix_memalign. Unfortunately that patch also disabled the  align_val_t
definition in C++17 as well.

This patch causes align_val_t to be exposed in C++17 regardless
of if we have the new/delete overloads.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292582 91177308-0d34-0410-b5e6-96231b3b80d8