[libcxx] adds a size-based representation for vector's 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.

This commit changes `std::vector`'s representation in a similar manner
to `__split_buffer` in #139632. We introduce a layout type that tracks
implementation details that differ between the pointer-based vector
and size-based vector representations. `vector` does not parameterise
its layout type, unlike `__split_buffer`. `__split_buffer` is used by
both `vector` and `deque`, and being able to take an ABI break on
`vector` doesn't automatically mean that users can also tolerate a break
on `deque`. The most convenient way to work around this problem is to
parameterise the layout type. Users of `std::vector` should not depend
on its layout, and libc++ has no internal reason to toggle it, so we
keep the representation concrete.

To use this feature, you'll need to configure libc++ so that it's built
with `-DLIBCXX_UNSTABLE_ABI=Yes`.

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

Author: cjdb