P0840R2: support for [[no_unique_address]] attribute

Summary:
Add support for the C++2a [[no_unique_address]] attribute for targets using the Itanium C++ ABI.

This depends on D63371.

Reviewers: rjmccall, aaron.ballman

Subscribers: dschuff, aheejin, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63451

llvm-svn: 363976