Summary:
This patch fixes llvm.org/PR35491 and LWG2157  (https://cplusplus.github.io/LWG/issue2157)

The fix attempts to maintain ABI compatibility by replacing the array with a instance of `aligned_storage`.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: lichray, cfe-commits

Differential Revision: https://reviews.llvm.org/D41223

llvm-svn: 324526