aarch64 ABI: revisit which ZST fields should be filtered from repr(C)

[pnkfelix]

Spawned off of #56877 (comment) and #57645 (comment)

The issues raised by #56877 were partially addressed by PR #57645 but there may be some followup work to do here.

Based on my current understanding, PR #57645 ended up settling on filtering out all ZST's when determining if a type is an instance of a so-called "homogeneous aggregate", and the remaining question is whether we have to deal with architecture/target dependencies when resolving the semantic question of whether the Rust type [T; 0] should map to the C typeT[0] or to T[].

Namely, it sounds like AArch64 may require us to restrict the filtering we do (for determining whether a type is a homogenous aggregate) to just the Rust ZST's like PhantomData.

Anyway, this issue is to tracking that leftover work to resolve that question.

[pnkfelix]

(This may be a good topic to hand off to the WG-ffi that should be forming soon. Cc @joshtriplett )

[VivekYadav7272]

Is this issue still active?