[clang] [lldb] [clang][RecordLayoutBuilder] Be stricter about inferring packed-ness in ExternalLayouts (PR #97443)

[Pavel Labath via cfe-commits]

labath wrote:

> (btw, @labath is the alignment miscalculation still causing issues for you internally? or did you find a workaround?)

The fire has been extinguished by #110648, but it's not ideal that any structure with a [[no_unique_address]] is treated as packed. It's not causing any issues right now, but I think we ought to fix this. (The change makes sense to me, but I don't feel it's my place to approve this)

https://github.com/llvm/llvm-project/pull/97443