[Concurrency] Reject top-level asynchronous code for now.

We haven't implemented any support for top-level async code yet, so
reject it in effects checking rather than crashing in the SIL verifier.
Fixes rdar://71512818.

Author: DougGregor

lib/Sema/TypeCheckEffects.cpp

@@ -985,8 +985,9 @@ class Context {
   }
 
   static Context forTopLevelCode(TopLevelCodeDecl *D) {
-    // Top-level code implicitly handles errors and 'async' calls.
-    return Context(/*handlesErrors=*/true, /*handlesAsync=*/true, None);
+    // Top-level code implicitly handles errors.
+    // TODO: Eventually, it will handle async as well.
+    return Context(/*handlesErrors=*/true, /*handlesAsync=*/false, None);
   }
 
   static Context forFunction(AbstractFunctionDecl *D) {

test/decl/var/async_let.swift

@@ -2,7 +2,10 @@
 
 // REQUIRES: concurrency
 
-async let x = 1 // okay
+func test() async {
+  async let x = 1 // okay
+  _ = await x
+}
 
 struct X {
   async let x = 1 // expected-error{{'async let' can only be used on local declarations}}

test/stmt/async.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
+
+// REQUIRES: concurrency
+
+func f() async -> Int { 0 }
+
+_ = await f() // expected-error{{'async' in a function that does not support concurrency}}
+
+async let y = await f() // expected-error{{'async let' in a function that does not support concurrency}}
+// expected-error@-1{{'async' in a function that does not support concurrency}}