Implement n3469 - constexpr for chrono

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187517 91177308-0d34-0410-b5e6-96231b3b80d8
Backwards!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187518 91177308-0d34-0410-b5e6-96231b3b80d8