[libcxx] Add <experimental/type_traits> for LFTS

Summary:
This adds the <experimental/type_traits> (minus invocation traits). Mostly just the `_v` traits.


Reviewers: K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5742

llvm-svn: 224626