Replace experimental-async flag with checking decl context

This patch replaces the use of the experimental-async-top-level flag
with checking the decl context of the top-level variable to determine
whether the top-level contexts should be async.

Author: etcwilde

lib/AST/Decl.cpp

@@ -736,8 +736,8 @@ bool Decl::preconcurrency() const {
   // Variables declared in top-level code are @_predatesConcurrency
   if (const VarDecl *var = dyn_cast<VarDecl>(this)) {
     const LangOptions &langOpts = getASTContext().LangOpts;
-    return !langOpts.isSwiftVersionAtLeast(6) &&
-           langOpts.EnableExperimentalAsyncTopLevel && var->isTopLevelGlobal();
+    return !langOpts.isSwiftVersionAtLeast(6) && var->isTopLevelGlobal() &&
+           var->getDeclContext()->isAsyncContext();
   }
 
   return false;
@@ -8946,9 +8946,8 @@ 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())
+    if (dc->isAsyncContext()) {
+      if (Type mainActor = dc->getASTContext().getMainActorType())
         return ActorIsolation::forGlobalActor(mainActor, /*unsafe=*/false);
     }
   }

lib/Sema/TypeCheckConcurrency.cpp

@@ -271,8 +271,7 @@ GlobalActorAttributeRequest::evaluate(
   } else if (auto storage = dyn_cast<AbstractStorageDecl>(decl)) {
     // Subscripts and properties are fine...
     if (auto var = dyn_cast<VarDecl>(storage)) {
-      if (var->isTopLevelGlobal() &&
-          var->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel) {
+      if (var->isTopLevelGlobal() && var->getDeclContext()->isAsyncContext()) {
         var->diagnose(diag::global_actor_top_level_var)
             .highlight(globalActorAttr->getRangeWithAt());
         return None;
@@ -3510,10 +3509,8 @@ ActorIsolation ActorIsolationRequest::evaluate(
   }
 
   if (auto var = dyn_cast<VarDecl>(value)) {
-    ASTContext &ctx = var->getASTContext();
-    if (var->isTopLevelGlobal() &&
-        ctx.LangOpts.EnableExperimentalAsyncTopLevel) {
-      if (Type mainActor = ctx.getMainActorType())
+    if (var->isTopLevelGlobal() && var->getDeclContext()->isAsyncContext()) {
+      if (Type mainActor = var->getASTContext().getMainActorType())
         return inferredIsolation(
             ActorIsolation::forGlobalActor(mainActor,
                                            /*unsafe=*/var->preconcurrency()));

lib/Sema/TypeCheckEffects.cpp

@@ -1492,9 +1492,7 @@ class Context {
   static Context forTopLevelCode(TopLevelCodeDecl *D) {
     // Top-level code implicitly handles errors.
     return Context(/*handlesErrors=*/true,
-                   /*handlesAsync=*/
-                   D->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel,
-                   None);
+                   /*handlesAsync=*/D->isAsyncContext(), None);
   }
 
   static Context forFunction(AbstractFunctionDecl *D) {