[clang] [CIR] Upstream support for record packing and padding (PR #136036)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm not sure I did a great job reviewing all the layout code, but didn't see anything of concern.  I'm somewhat fearful that we're pulling in existing "scary" code from Classic-CodeGen that is a 'thar be dragons' though.

At the same time, re-engineering this at this time also seems scary as well, so I think we have to live with it for now?  Anyway, please let others review this before merging.

https://github.com/llvm/llvm-project/pull/136036