Port TypeScript#62676: Fix slow sync iteration types spoiling cached async iteration types

[Copilot]

When a type implements both Iterable<T_Sync> and AsyncIterable<T_Async>, the sync slow resolution path could cache results before the async slow path ran, causing for await loops to incorrectly yield the sync type parameter instead of the async one.

type Stream<T_Sync, T_Async> = Iterable<T_Sync> & AsyncIterable<T_Async>;

async function loop(stream: Stream<A, B>) {
  for await (const b of stream) {} // b was incorrectly typed as A instead of B
}

• Move getIterationTypesOfIterableSlow calls inside their respective fast-check blocks so async slow resolution always completes before sync slow resolution runs, preventing cache poisoning

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[jakebailey]

Hm, this does nothing. Maybe we avoid the problem entirely now?

[jakebailey]

microsoft/TypeScript#62676

Probably worth pulling, just because this probably is an ordering thing we now just so happen to avoid in this case, but not others

[Copilot]

Pull request overview

Ports TypeScript#62676 to the Go checker to ensure for await...of prefers AsyncIterable iteration types over sync fallbacks, avoiding incorrect cached results when a type supports both sync and async iteration.

Changes:

• Reorders getIterationTypesOfIterableWorker resolution so async slow-path resolution completes before any sync-path fallback.
• Shares a single diags slice across the async/sync slow paths for related diagnostic info.