[libc++] Fix capacity increase issue with `shrink_to_fit` for `__split_buffer` (#117720)

This PR fixes the issue where `__split_buffer::shrink_to_fit` may
unexpectedly increase the capacity, similar to the issue for
`std::vector` in #97895. The fix follows the same approach
used in #97895 for `std::vector`.

GitOrigin-RevId: 8458bbe5947db75041579c7c1a0f7f79c8e992d9
1 file changed