Detach enable-experimental-async-top-level flag

SE-0343 is approved so it's time to pull the feature out from behind the
experimental feature flag. This patch pulls it out and deprecates
passing the flag to the frontend so that we can pull it out entirely
eventually.

Author: etcwilde

include/swift/AST/DiagnosticsFrontend.def

@@ -420,6 +420,8 @@ ERROR(error_nonexistent_output_dir,none,
 REMARK(interface_file_backup_used,none,
       "building module from '%0' failed; retrying building module from '%1'", (StringRef, StringRef))
 
+WARNING(warn_flag_deprecated,none, "flag '%0' is deprecated", (StringRef))
+
 // Dependency Verifier Diagnostics
 ERROR(missing_member_dependency,none,
       "expected "

include/swift/Basic/LangOptions.h

@@ -407,9 +407,6 @@ namespace swift {
     /// cases.
     bool EnableNonFrozenEnumExhaustivityDiagnostics = false;
 
-    /// Enable making top-level code support concurrency
-    bool EnableExperimentalAsyncTopLevel = false;
-
     /// Regex for the passes that should report passed and missed optimizations.
     ///
     /// These are shared_ptrs so that this class remains copyable.

lib/AST/Decl.cpp

@@ -9088,9 +9088,7 @@ ActorIsolation swift::getActorIsolationOfContext(DeclContext *dc) {
   }
 
   if (auto *tld = dyn_cast<TopLevelCodeDecl>(dc)) {
-    if (dc->isAsyncContext() ||
-        (dc->getASTContext().LangOpts.WarnConcurrency &&
-         dc->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel)) {
+    if (dc->isAsyncContext() || dc->getASTContext().LangOpts.WarnConcurrency) {
       if (Type mainActor = dc->getASTContext().getMainActorType())
         return ActorIsolation::forGlobalActor(mainActor, /*unsafe=*/false);
     }

lib/AST/DeclContext.cpp

@@ -1239,12 +1239,10 @@ bool DeclContext::isAsyncContext() const {
     return false;
   case DeclContextKind::FileUnit:
     if (const SourceFile *sf = dyn_cast<SourceFile>(this))
-      return getASTContext().LangOpts.EnableExperimentalAsyncTopLevel &&
-             sf->isAsyncTopLevelSourceFile();
+      return sf->isAsyncTopLevelSourceFile();
     return false;
   case DeclContextKind::TopLevelCodeDecl:
-    return getASTContext().LangOpts.EnableExperimentalAsyncTopLevel &&
-           getParent()->isAsyncContext();
+    return getParent()->isAsyncContext();
   case DeclContextKind::AbstractClosureExpr:
     return cast<AbstractClosureExpr>(this)->isBodyAsync();
   case DeclContextKind::AbstractFunctionDecl: {

lib/Frontend/CompilerInvocation.cpp

@@ -487,16 +487,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);
 
-  Opts.EnableExperimentalAsyncTopLevel |=
-    Args.hasArg(OPT_enable_experimental_async_top_level);
-
   /// experimental distributed also implicitly enables experimental concurrency
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_distributed);
-  Opts.EnableExperimentalConcurrency |=
-    Args.hasArg(OPT_enable_experimental_async_top_level);
+
+  if (Args.hasArg(OPT_enable_experimental_async_top_level))
+    Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
+                   "-enable-experimental-async-top-level");
 
   Opts.DiagnoseInvalidEphemeralnessAsError |=
       Args.hasArg(OPT_enable_invalid_ephemeralness_as_error);

lib/Sema/TypeCheckConcurrency.cpp

@@ -344,14 +344,14 @@ GlobalActorAttributeRequest::evaluate(
     if (auto var = dyn_cast<VarDecl>(storage)) {
 
       // ... but not if it's an async-context top-level global
-      if (var->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel &&
-          var->isTopLevelGlobal() && (var->getDeclContext()->isAsyncContext()
-            || var->getASTContext().LangOpts.WarnConcurrency)) {
+      if (var->isTopLevelGlobal() &&
+          (var->getDeclContext()->isAsyncContext() ||
+           var->getASTContext().LangOpts.WarnConcurrency)) {
         var->diagnose(diag::global_actor_top_level_var)
             .highlight(globalActorAttr->getRangeWithAt());
         return None;
       }
-      
+
       // ... and not if it's local property
       if (var->getDeclContext()->isLocalContext()) {
         var->diagnose(diag::global_actor_on_local_variable, var->getName())
@@ -3869,8 +3869,7 @@ ActorIsolation ActorIsolationRequest::evaluate(
   }
 
   if (auto var = dyn_cast<VarDecl>(value)) {
-    if (var->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel &&
-        var->isTopLevelGlobal() &&
+    if (var->isTopLevelGlobal() &&
         (var->getASTContext().LangOpts.WarnConcurrency ||
          var->getDeclContext()->isAsyncContext())) {
       if (Type mainActor = var->getASTContext().getMainActorType())