ir: union: support #[repr(align(...))] and #[repr(packed)]
ir: union: support #[repr(align(...))] and #[repr(packed)]

Users configure the annotations for this feature using the same [layout]
settings as for structures (because -- unlike #[must_use] -- all
well-known compilers use the same annotations for all types).

Signed-off-by: Aleksa Sarai <b305c9b0212d6145eea67bbffd24e29cb79c759c@cyphar.com>

eqrion/cbindgen,eqrion/cbindgen,eqrion/cbindgen