[Actor isolation] Remove pointless walk from getInnermostIsolatedContext().

This method was walking up the context stack to determine isolation of
a context, which is better handled by existing facilities.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -1544,55 +1544,21 @@ namespace {
     }
 
     /// Get the actor isolation of the innermost relevant context.
-    ActorIsolation getInnermostIsolatedContext(const DeclContext *constDC) {
-      // Retrieve the actor isolation for a declaration somewhere in our
-      // declaration context chain and map it into our own context so that
-      // the types can be compared.
-      auto getActorIsolation = [constDC](ValueDecl *value) {
-        switch (auto isolation = swift::getActorIsolation(value)) {
-        case ActorIsolation::ActorInstance:
-        case ActorIsolation::Independent:
-        case ActorIsolation::IndependentUnsafe:
-        case ActorIsolation::Unspecified:
-          return isolation;
-
-        case ActorIsolation::GlobalActor:
-        case ActorIsolation::GlobalActorUnsafe:
-          return ActorIsolation::forGlobalActor(
-              constDC->mapTypeIntoContext(isolation.getGlobalActor()),
-              isolation == ActorIsolation::GlobalActorUnsafe);
-        }
-      };
-
-      auto dc = const_cast<DeclContext *>(constDC);
-      while (!dc->isModuleScopeContext()) {
-        if (auto closure = dyn_cast<AbstractClosureExpr>(dc))
-          return getActorIsolationOfContext(dc);
-
-        // Functions have actor isolation defined on them.
-        if (auto func = dyn_cast<AbstractFunctionDecl>(dc))
-          return getActorIsolation(func);
-
-        // Subscripts have actor isolation defined on them.
-        if (auto subscript = dyn_cast<SubscriptDecl>(dc))
-          return getActorIsolation(subscript);
-
-        // Pattern binding declarations have actor isolation defined on their
-        // properties, if any.
-        if (auto init = dyn_cast<PatternBindingInitializer>(dc)) {
-          auto var = init->getBinding()->getAnchoringVarDecl(
-              init->getBindingIndex());
-          if (var)
-            return getActorIsolation(var);
-
-          return ActorIsolation::forUnspecified();
-        }
+    ActorIsolation getInnermostIsolatedContext(DeclContext *dc) {
+      // Retrieve the actor isolation of the context.
+      switch (auto isolation = getActorIsolationOfContext(dc)) {
+      case ActorIsolation::ActorInstance:
+      case ActorIsolation::Independent:
+      case ActorIsolation::IndependentUnsafe:
+      case ActorIsolation::Unspecified:
+        return isolation;
 
-        return ActorIsolation::forUnspecified();
+      case ActorIsolation::GlobalActor:
+      case ActorIsolation::GlobalActorUnsafe:
+        return ActorIsolation::forGlobalActor(
+            dc->mapTypeIntoContext(isolation.getGlobalActor()),
+            isolation == ActorIsolation::GlobalActorUnsafe);
       }
-
-      // At module scope, actor independence with safety is assumed.
-      return ActorIsolation::forIndependent(ActorIndependentKind::Safe);
     }
 
     bool isInAsynchronousContext() const {
@@ -1704,7 +1670,7 @@ namespace {
         bool isCrossActor,
         Expr *context) {
       ValueDecl *value = valueRef.getDecl();
-      auto declContext = getDeclContext();
+      auto declContext = const_cast<DeclContext *>(getDeclContext());
 
       // Check whether we are within the same isolation context, in which
       // case there is nothing further to check,