rustc_target/loongarch: Fix passing of transparent unions with only one non-ZST member

[heiher]

This ensures that MaybeUninit<T> has the same ABI as T when passed through an extern "C" function.

Fixes #115509

r? @bjorn3

[bjorn3]

@bors r+

[bors]

📌 Commit 10d55c3 has been approved by bjorn3

It is now in the queue for this repository.

[bors]

⌛ Testing commit 10d55c3 with merge ad36b5a...

[bors]

☀️ Test successful - checks-actions
Approved by: bjorn3
Pushing ad36b5a to master...

[rust-timer]

Finished benchmarking commit (ad36b5a): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.0%	[5.0%, 5.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-3.2%, -1.3%]	3
All ❌✅ (primary)	5.0%	[5.0%, 5.0%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 633.382s -> 632.887s (-0.08%)
Artifact size: 317.82 MiB -> 317.80 MiB (-0.00%)

[RalfJung]

This should really share code with the riscv ABI, having this all implemented twice is a pain. The ABIs are 100% identical, at least in their current Rust implementation.