[libc++] Fix PR22922 - Allocator support for std::function does not know how to rebind.

Summary:
This patch changes std::function to use allocator_traits to rebind the allocator instead of allocator itself.

It also changes most of the tests to use `bare_allocator` where possible instead of `test_allocator`.

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8391

llvm-svn: 232686