[SE-0371] Back-deploy support for main-actor-isolated deinit

When targeting a platform that predates the introduction of isolated
deinit, make a narrow exception that allows main-actor-isolated deinit
to work through a special, inlineable entrypoint that is
back-deployed. This implementation

  1. Calls into the real implementation when available, otherwise
  2. Checks if we're on the main thread, destroying immediately when
we are, otherwise
  3. Creates a new task on the main actor to handle destruction.

This implementation is less efficient than the implementation in the
runtime, but allows us to back-deploy this functionality as far back
as concurrency goes.

Fixes rdar://151029118.

include/swift/Runtime/RuntimeFunctions.def

@@ -2396,13 +2396,25 @@ FUNCTION(TaskSwitchFunc,
 //                                  size_t flags);
 FUNCTION(DeinitOnExecutorFunc,
          _Concurrency, swift_task_deinitOnExecutor, SwiftCC,
-         ConcurrencyAvailability,
+         IsolatedDeinitAvailability,
          RETURNS(VoidTy),
          ARGS(Int8PtrTy, Int8PtrTy, ExecutorFirstTy, ExecutorSecondTy, SizeTy),
          ATTRS(NoUnwind),
          EFFECT(RuntimeEffect::Concurrency),
          UNKNOWN_MEMEFFECTS)
 
+// void swift_task_deinitOnExecutorMainActorBackDeploy(void *object,
+//                                  DeinitWorkFunction *work,
+//                                  SerialExecutorRef newExecutor,
+//                                  size_t flags);
+FUNCTION(DeinitOnExecutorMainActorBackDeployFunc,
+         _Concurrency, deinitOnExecutorMainActorBackDeploy, SwiftCC,
+         ConcurrencyAvailability,
+         RETURNS(VoidTy),
+         ARGS(Int8PtrTy, Int8PtrTy, ExecutorFirstTy, ExecutorSecondTy, SizeTy),
+         ATTRS(NoUnwind),
+         EFFECT(RuntimeEffect::Concurrency),
+         UNKNOWN_MEMEFFECTS)
 
 // AsyncTask *swift_continuation_init(AsyncContext *continuationContext,
 //                                    AsyncContinuationFlags);