Copy byval argument to local stackslot if alignment is insufficient

[0xmuon]

fix: #1465

Copy the underaligned byval (indirect) arguments into a local stackslot when the incoming pointer alignment is less than the Rust ABI alignment. This avoids miscompiles from assuming stronger alignment than the ABI guarantees.

[bjorn3]

Maybe pull out the Option around the field ty and have:

Suggested change

	Normal(ArgValue<'tcx>),
	Spread(Vec<ArgValue<'tcx>>),
	Normal(Option<ArgValue<'tcx>>),
	Spread(Vec<Option<ArgValue<'tcx>>>),

? The align field is unused when the argument is not passed at all. I don't know how much that would complicate cvalue_for_param though.

View changes since the review

[0xmuon]

I considered that, but since cvalue_for_param now returns a single ArgValue, keeping Option outside would make the call sites and caller-location path a bit more awkward again. Since the underaligned flag is now just a bool, I kept ArgKind::{Normal,Spread} holding ArgValue directly and use value: None for PassMode::Ignore.