[libcxx] adds size-based __split_buffer representation to unstable ABI (#139632)

**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]: https://discourse.llvm.org/t/adding-a-size-based-vector-to-libc-s-unstable-abi/86306

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Co-authored-by: Jorge Gorbe Moya <[email protected]>

Author: cjdb