[libcxx] expose experimental::erased_type for all standard versions.

Summary: The polymorphic allocator implementation would greatly benefit by defining virtual functions in the dynlib instead of inline. In order to do that some types are going to have to be available outside of c++1y. This is the first step.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4554