[Distributed] Use distributed thunk witnesses for DA protocol extensions.

When a distributed member of an extension of a DistributedActor
protocol is used to satisfy a distributed requirement for the same
protocol, make sure we still use the distributed thunk.

Noticed by inspection.

Author: DougGregor

lib/SILGen/SILGenType.cpp

@@ -707,10 +707,6 @@ SILFunction *SILGenModule::emitProtocolWitness(
        !requirement.getFuncDecl()->isDistributed() &&
        witnessRef.hasDecl() && witnessRef.getFuncDecl() &&
        witnessRef.getFuncDecl()->isDistributed());
-  // We only need to use thunks when we go cross-actor:
-  shouldUseDistributedThunkWitness = shouldUseDistributedThunkWitness &&
-                                     getActorIsolation(requirement.getDecl()) !=
-                                         getActorIsolation(witness.getDecl());
   if (shouldUseDistributedThunkWitness) {
     auto thunkDeclRef = SILDeclRef(
         witnessRef.getFuncDecl()->getDistributedThunk(),

test/SILGen/distributed_thunk.swift

@@ -59,3 +59,29 @@ distributed actor DA4: ServerProto {
   // CHECK: function_ref @$s17distributed_thunk3DA4C11doSomethingyyYaKFTE
   distributed func doSomething() throws { }
 }
+
+protocol Server2: DistributedActor {
+  distributed func sayHello()
+}
+
+extension Server2 where ActorSystem == LocalTestingDistributedActorSystem {
+  // CHECK-LABEL: sil hidden [thunk] [distributed] [ref_adhoc_requirement_witness "$s11Distributed29LocalTestingInvocationDecoderC18decodeNextArgumentxyKSeRzSERzlF"] [ossa] @$s17distributed_thunk7Server2PAA11Distributed012LocalTestingD11ActorSystemC0gH0RtzrlE8sayHelloyyYaKFTE
+  // CHECK-NOT: return
+  // CHECK: function_ref @swift_distributed_actor_is_remote
+  distributed func sayHello() { /* default impl */ }
+
+  // CHECK-LABEL: sil hidden [distributed] [ossa] @$s17distributed_thunk7Server2PAA11Distributed012LocalTestingD11ActorSystemC0gH0RtzrlE8sayHelloyyF
+  // CHECK-NOT: swift_distributed_actor_is_remote
+  // CHECK: return
+}
+
+distributed actor DA5: Server2 {
+  typealias ActorSystem = LocalTestingDistributedActorSystem
+  // uses default impl
+
+  // CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s17distributed_thunk3DA5CAA7Server2A2aDP8sayHelloyyFTW
+  // CHECK: function_ref @$s17distributed_thunk7Server2PAA11Distributed012LocalTestingD11ActorSystemC0gH0RtzrlE8sayHelloyyF
+
+  // CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s17distributed_thunk3DA5CAA7Server2A2aDP8sayHelloyyFTWTE
+  // CHECK: function_ref @$s17distributed_thunk7Server2PAA11Distributed012LocalTestingD11ActorSystemC0gH0RtzrlE8sayHelloyyYaKFTE
+}