Fix top-level crash/missing diagnostic

This patch fixes the crash on assertion builds and the missing
diagnostic on normal builds when operating on declarations in the
top-level context.

Author: etcwilde

lib/Sema/TypeCheckAttr.cpp

@@ -2733,6 +2733,10 @@ static void lookupReplacedDecl(DeclNameRef replacedDeclName,
                                SmallVectorImpl<ValueDecl *> &results) {
   auto *declCtxt = replacement->getDeclContext();
 
+  // Hop up to the FileUnit if we're in top-level code
+  if (auto *toplevel = dyn_cast<TopLevelCodeDecl>(declCtxt))
+    declCtxt = toplevel->getDeclContext();
+
   // Look at the accessors' storage's context.
   if (auto *accessor = dyn_cast<AccessorDecl>(replacement)) {
     auto *storage = accessor->getStorage();

test/attr/attr_availability_async_rename_toplevel.swift

@@ -0,0 +1,22 @@
+// REQUIRES: concurrency
+
+// RUN: %target-typecheck-verify-swift
+
+if #available(macOS 12.0, *) {
+  @available(*, renamed: "process(data:)")
+  func process(data: [Int], completion: @escaping ([Int]) -> Void) { completion(data) }
+  // expected-note@+1{{'process(data:)' declared here}}
+  func process(data: [Int]) async -> [Int] { return data }
+
+  func asyncFunc(data: [Int]) async {
+    defer {
+      process(data: data, completion: { print($0) })
+    }
+
+    func b() {
+      process(data: data, completion: { print($0) })
+    }
+    // expected-warning@+1{{consider using asynchronous alternative function}}
+    process(data: data, completion: { print($0) })
+  }
+}