Teach witness thunks to hop to the actor when needed.

When a synchronous, actor-isolated declaration witnesses an
asynchronous, not-similarly-isolated requirement, emit an actor hop
within the witness thunk to ensure that we properly enter the context
of the actor.

Fixes #58517 / rdar://92881539.

(cherry picked from commit 25a7988)

Author: DougGregor

lib/SILGen/SILGenPoly.cpp

@@ -4505,6 +4505,25 @@ void SILGenFunction::emitProtocolWitness(
                                          witnessUnsubstTy->getSelfParameter());
   }
 
+  // If we are supposed to hop to the actor, do so now.
+  if (enterIsolation) {
+    Optional<ManagedValue> actorSelf;
+    if (*enterIsolation == ActorIsolation::ActorInstance) {
+      auto actorSelfVal = origParams.back();
+
+      if (actorSelfVal.getType().isAddress()) {
+        auto &actorSelfTL = getTypeLowering(actorSelfVal.getType());
+        if (!actorSelfTL.isAddressOnly()) {
+          actorSelfVal = emitManagedLoad(*this, loc, actorSelfVal, actorSelfTL);
+        }
+      }
+
+      actorSelf = actorSelfVal;
+    }
+
+    emitHopToTargetActor(loc, enterIsolation, actorSelf);
+  }
+
   // For static C++ methods and constructors, we need to drop the (metatype)
   // "self" param. The "native" SIL representation will look like this:
   //    @convention(method) (@thin Foo.Type) -> () but the "actual" SIL function