Top-level code is executed on the main actor

This patch sets the actor isolation of the top-level code contexts to be
on the main actor. This holds since the implicit `async_Main` function
generated is executed on the main actor executor.

Author: etcwilde

lib/AST/Decl.cpp

@@ -8949,6 +8949,14 @@ ActorIsolation swift::getActorIsolationOfContext(DeclContext *dc) {
     }
   }
 
+  if (auto *tld = dyn_cast<TopLevelCodeDecl>(dc)) {
+    ASTContext &ctx = dc->getASTContext();
+    if (ctx.LangOpts.EnableExperimentalAsyncTopLevel) {
+      if (Type mainActor = ctx.getMainActorType())
+        return ActorIsolation::forGlobalActor(mainActor, /*unsafe=*/false);
+    }
+  }
+
   return ActorIsolation::forUnspecified();
 }
 

lib/Sema/TypeCheckConcurrency.h

@@ -199,6 +199,8 @@ class ActorIsolationRestriction {
   }
 
   /// Determine the isolation rules for a given declaration.
+  /// The isolation restriction represents the isolation requirement of the
+  /// using context.
   ///
   /// \param fromExpression Indicates that the reference is coming from an
   /// expression.