[libcxx] Add std::any
This patch adds std::any by moving/adapting <experimental/any>.
This patch also implements the std::any parts of p0032r3 (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0032r3.pdf)
and LWG 2509 (http://cplusplus.github.io/LWG/lwg-defects.html#2509).
I plan to push it in a day or two if there are no comments.
Reviewers: mclow.lists, EricWF
Differential Revision: https://reviews.llvm.org/D22733