[5.10🍒] autoreleasing isolated foreign errors

[kavon]

• Explanation: An @autoreleasing value returned from an async function is not being retained prior to changing executors/actors, only afterwards. If the caller and callee have different actors, then an executor hop will happen, which triggers the autorelease pool and thus destroying the returned value. Upon returning from the hop, we will attempt to retain the destroyed value and trigger a use-after-free / crash in the runtime due to incrementing a ref count of a destroyed object. Here's a visual of the change:

// Bad, prior situation:
let autoreleasingResult = await call(...)
hop_to_executor caller_actor
retain autoreleasingResult

// Corrected:
let autoreleasingResult = await call(...)
retain autoreleasingResult
hop_to_executor caller_actor

• Radars/Issues: rdar://114049646
• Scope: Limited to just SILGen and basically just delays the emission of the hop-back until immediately after we retrieve the results and perform clean-ups on arguments after an async call.
• Risk: Medium. It's a change to SILGen that will slightly perturb the SIL for many async calls (namely, an end_borrow that normally is right after the hop now comes before it), though in most cases it should lead to no changes in the final assembly.
• Testing: regression test included, in addition to manual testing.
  Reviewed By: @gottesmm
• main PR: [SILGen] avoid hop before autoreleased foreign error is retained #68415

[gottesmm]

LGTM.

[kavon]

@swift-ci please test