Support overaligned types in `aligned_storage`.

Summary:
The current implementation of aligned storage was written before we had `alignas`, so it used a list of builtin types to force the alignment. But this doesn't work overaligned requests.

This patch adds a fallback case supporting over-alignment. It only affects case that were previously ill-formed.

Reviewers: rsmith, ldionne, dlj, mclow.lists

Reviewed By: mclow.lists

Subscribers: mclow.lists, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D61301

llvm-svn: 359596