[libcxx] adds size-based __split_buffer representation to unstable ABI

**tl;dr** We can significantly improve the runtime performance of
`std::vector` by changing its representation from three pointers to one
pointer and two integers. This document explains the details of this
change, along with the justifications for making it. See the [RFC] for
more information.

`vector` depends on `__split_buffer` for inserting elements. Changing
`__split_buffer` to match `vector`'s representation simplifies the
model, as it eliminates the need to convert between two different
representations of a contiguous buffer in the same configuration of
libc++.

[RFC]: TODO

Author: cjdb