[AST] Move conversion of ClosureActorIsolation to ActorIsolation to its own function

Just a little bit of cleanup because this logic is really quite general and doesn't need to live in `getActorIsolationOfContext`.

Author: ahoppen

include/swift/AST/Expr.h

@@ -3670,6 +3670,8 @@ class ClosureActorIsolation {
   bool preconcurrency() const {
     return storage.getInt();
   }
+
+  ActorIsolation getActorIsolation() const;
 };
 
 /// A base class for closure expressions.

lib/AST/Decl.cpp

@@ -9215,26 +9215,7 @@ ActorIsolation swift::getActorIsolationOfContext(DeclContext *dc) {
 
 
   if (auto *closure = dyn_cast<AbstractClosureExpr>(dc)) {
-    switch (auto isolation = closure->getActorIsolation()) {
-    case ClosureActorIsolation::Independent:
-      return ActorIsolation::forIndependent()
-                .withPreconcurrency(isolation.preconcurrency());
-
-    case ClosureActorIsolation::GlobalActor: {
-      return ActorIsolation::forGlobalActor(
-          isolation.getGlobalActor(), /*unsafe=*/false)
-                .withPreconcurrency(isolation.preconcurrency());
-    }
-
-    case ClosureActorIsolation::ActorInstance: {
-      auto selfDecl = isolation.getActorInstance();
-      auto actor = selfDecl->getType()->getReferenceStorageReferent()
-          ->getAnyActor();
-      assert(actor && "Bad closure actor isolation?");
-      return ActorIsolation::forActorInstance(actor)
-        .withPreconcurrency(isolation.preconcurrency());
-    }
-    }
+    return closure->getActorIsolation().getActorIsolation();
   }
 
   if (auto *tld = dyn_cast<TopLevelCodeDecl>(dc)) {

lib/AST/Expr.cpp

@@ -1737,6 +1737,28 @@ RebindSelfInConstructorExpr::getCalledConstructor(bool &isChainToSuper) const {
   return otherCtorRef;
 }
 
+ActorIsolation ClosureActorIsolation::getActorIsolation() const {
+  switch (getKind()) {
+  case ClosureActorIsolation::Independent:
+    return ActorIsolation::forIndependent().withPreconcurrency(
+        preconcurrency());
+
+  case ClosureActorIsolation::GlobalActor: {
+    return ActorIsolation::forGlobalActor(getGlobalActor(), /*unsafe=*/false)
+        .withPreconcurrency(preconcurrency());
+  }
+
+  case ClosureActorIsolation::ActorInstance: {
+    auto selfDecl = getActorInstance();
+    auto actor =
+        selfDecl->getType()->getReferenceStorageReferent()->getAnyActor();
+    assert(actor && "Bad closure actor isolation?");
+    return ActorIsolation::forActorInstance(actor).withPreconcurrency(
+        preconcurrency());
+  }
+  }
+}
+
 void AbstractClosureExpr::setParameterList(ParameterList *P) {
   parameterList = P;
   // Change the DeclContext of any parameters to be this closure.