[libc++] Simplify aligned_storage (#114665)
The main template of `aligned_storage` is only ever used when we have
extremely overaligned types (> 16384), so we effectively only ever use
the specializations currently. This means that we only instantiate the
main template for overaligned types. Instead of doing this dance, we can
just define the main template to use `_ALIGNAS`, just like the
specializations. This makes the implementation of `aligned_storage`
significantly less confusing.
