[libcxx] Add <experimental/chrono>

Summary:
This patch adds <experimental/chrono> which only contains a single variable template.

See: https://rawgit.com/cplusplus/fundamentals-ts/v1/fundamentals-ts.html#time

Reviewers: jroelofs, danalbert, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7352

llvm-svn: 227860