[clang] Respect field alignment in layout compatibility of structs (#84313)

This patch implements
[CWG2586](https://cplusplus.github.io/CWG/issues/2583.html) "Common
initial sequence should consider over-alignment". Note that alignment of
union members doesn't have to match, as layout compatibility of unions
is not defined in terms of common initial sequence
(http://eel.is/c++draft/class.mem.general#25).