[libcxx] Add <experimental/utility> header for LFTS.

Summary:
This patch adds the `<experimental/utility>` header as specified in the latest draft of the library fundamentals TS.

`<experimental/utility>` only contains `class erased_type`.

This patch also updates the documentation to list the `erased_type` class as "initial implementation complete".

Test Plan:
Three test cases where added:

1. Test that `_LIBCPP_VERSION` is defined.
2. Test that `<utility>` has been included.
3. Test that `erased_type` is in the correct namespace and is constexpr default constructible.

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4510