Make TopLevelCode async context

This patch makes the FileUnit and TopLevelCodeDecl contexts asynchronous
contexts when the `-enable-experimental-async-top-level` flag is passed.

Author: etcwilde

include/swift/AST/Decl.h

@@ -2015,6 +2015,9 @@ class TopLevelCodeDecl : public DeclContext, public Decl {
   SourceLoc getStartLoc() const;
   SourceRange getSourceRange() const;
 
+  LLVM_READONLY
+  ASTContext &getASTContext() const { return DeclContext::getASTContext(); }
+
   static bool classof(const Decl *D) {
     return D->getKind() == DeclKind::TopLevelCode;
   }

lib/AST/DeclContext.cpp

@@ -1211,14 +1211,15 @@ bool DeclContext::isClassConstrainedProtocolExtension() const {
 bool DeclContext::isAsyncContext() const {
   switch (getContextKind()) {
   case DeclContextKind::Initializer:
-  case DeclContextKind::TopLevelCodeDecl:
   case DeclContextKind::EnumElementDecl:
   case DeclContextKind::ExtensionDecl:
   case DeclContextKind::SerializedLocal:
   case DeclContextKind::Module:
-  case DeclContextKind::FileUnit:
   case DeclContextKind::GenericTypeDecl:
     return false;
+  case DeclContextKind::FileUnit:
+  case DeclContextKind::TopLevelCodeDecl:
+    return getASTContext().LangOpts.EnableExperimentalAsyncTopLevel;
   case DeclContextKind::AbstractClosureExpr:
     return cast<AbstractClosureExpr>(this)->isBodyAsync();
   case DeclContextKind::AbstractFunctionDecl: {

lib/Sema/TypeCheckEffects.cpp

@@ -1491,8 +1491,10 @@ class Context {
 
   static Context forTopLevelCode(TopLevelCodeDecl *D) {
     // Top-level code implicitly handles errors.
-    // TODO: Eventually, it will handle async as well.
-    return Context(/*handlesErrors=*/true, /*handlesAsync=*/false, None);
+    return Context(/*handlesErrors=*/true,
+                   /*handlesAsync=*/
+                   D->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel,
+                   None);
   }
 
   static Context forFunction(AbstractFunctionDecl *D) {