104/104 regression tests pass. The Triple round-trip
(triple_imp writes {11, 22, 33} on the IMP side → #objc_call(Triple)
reads them back) is the test of record.
emit_llvm.zig changes:
1. `objc_msg_send` arm — when `needsByval(ret_ty)` (same predicate
the plain-foreign-call path uses), apply the sret transform:
- ret type collapses to void
- prepend a `ptr` param at index 0 (call site provides an
alloca slot)
- mirror `sret(<RetType>)` on the call site so the AArch64 x8
/ SysV-AMD64 hidden-ptr ABI lowers correctly
- load the result from the slot post-call
The IR shape now matches clang exactly:
call void @objc_msgSend(ptr sret({...}) %slot, ptr %recv, ptr %sel)
2. `.ret` arm — the body-side counterpart for sx fns whose declared
return type is sret-shaped (sx-defined IMPs registered via
`class_addMethod` produce these). When the current function's
`needsByval(func.ret)` predicate holds, store the IR ret value
through the prepended sret slot (param 0) and emit `ret void`.
Previously the unconditional coerceArg path turned the struct
value into `undef` and emitted `ret void undef` — illegal LLVM.
Test mechanics: registers `SxTripleProbe : NSObject` at runtime via
`objc_allocateClassPair` + `class_addMethod`, IMP returns
Triple{11, 22, 33}. `#objc_call(Triple)(instance, "tripleValue")`
gets them back, round-trip pinned in the .txt snapshot and the
IR-shape snapshot.
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