GCC ABI Compatibility: Preserve alignment of non-pod members in packed structs

This matches GCC: https://godbolt.org/z/sM5q95PGY

I realize this is an API break for clang+clang - so I'm totally open to
discussing how we should deal with that. If Apple wants to keep the
Clang layout indefinitely, if we want to put a flag on this so non-Apple
folks can opt out of this fix/new behavior.

Differential Revision: https://reviews.llvm.org/D117616