Distinguish repr(C) ZSTs from others in ABI compatibility rules

[Jules-Bertholet]

(Split out from compiler implementation in #156112)

Some C ABIs pass and return ZSTs by pointer. But () should never be returned by pointer, as it must match void. To account for this, we have to weaken the present guarantee of "any two types with size 0 and alignment 1 are ABI-compatible" to exclude repr(C).

Fixes rust-lang/unsafe-code-guidelines#552; see also #78586, #155299.

@rustbot label T-lang A-ABI needs-fcp

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r? @Mark-Simulacrum

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[Jules-Bertholet]

We exempt arrays for 2 reasons:

• A future version of C, or some other language we would like to do FFI with, might allow passing arrays to functions directly by value, in a way that would conflict with these guarantees
• It would be nice to also use "trivial ABI" in the specification of repr(C), and we need this clause for that. See discussion at repr(ordered_fields) rfcs#3845 (comment)

We could also simplify this clause by saying merely:

Suggested change

	/// - Not an array whose element type fails to satisfy these conditions
	/// - Not an array

The downside would be a larger breaking change.

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