Implement is_always_lock_free

Summary:

This was voted into C++17 at the Jacksonville meeting. The final P0152R1
paper will be in the upcoming post-Jacksonville mailing, and is also
available here:

  http://jfbastien.github.io/papers/P0152R1.html

Reviewers: mclow.lists, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D17951

llvm-svn: 264413