Summary:
This patch implements the library side of P0035R4. The implementation is thanks to @rsmith.

In addition to the C++17 implementation, the library implementation can be explicitly turned on using `-faligned-allocation` in all dialects.


Reviewers: mclow.lists, rsmith

Subscribers: rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D25591

llvm-svn: 284206