Follow up to currently-reverted https://github.com/swiftlang/swift/pull/89416.
On wasm32 the WebAssembly backend pads the `swiftself` and `swifterror` parameters, which left the indirect typed-error pointer and the `swiftself`/context slot at different LLVM-IR positions for Thin versus Thick async callers; caller and callee then disagreed on which trailing pointer held the typed-error slot and miscompiled async typed-throws calls (#89155: `Result.init(catching:) async` trapped in a value-witness copy on Wasm).
With this change `SignatureExpansion::expandAsyncEntryType` now emits `addIndirectThrowingResult` before the `swiftself`/context slot, so the trailing pair is always `[ind_error, swiftself]` and Thin and Thick agree on the position regardless of parameter padding. The result-shape decision moves behind one gate, `hasTrailingAsyncErrorContextPair` (`GenCall.h`), consumed by the three sites that read these slots positionally: `AsyncCallEmission::setFromCallee`, `AsyncPartialApplicationForwarderEmission`, and `AsyncNativeCCEntryPointArgumentEmission::mapAsyncParameters` (the last also pre-extracts the witness-method Self/witness-table that trail the pair).
Each consumer carries an assertion that fires if the gate ever diverges from the slots actually filled.
Fixes#89320.
Max Desiatov
4c990823cd