[llvm] Add -Wms-bitfield-padding warning when possible (PR #139828)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

This is a draft PR as it might produce a bunch of noise, I've got a PR to fix the cases I can repeat locally: https://github.com/llvm/llvm-project/pull/139825

I suspect issues may occur in other configs and I'd like to hear from others before making this a non-draft PR

https://github.com/llvm/llvm-project/pull/139828