Move test/IRGen/async/throwing.swift to async-main

This patch migrates this test off of using runAsyncAndBlock so that we
can eventually delete that function.

Author: etcwilde

test/IRGen/async/throwing.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift -Xfrontend -enable-experimental-concurrency -g %s -module-name main -o %t/main
+// RUN: %target-build-swift -parse-as-library -Xfrontend -enable-experimental-concurrency -g %s -module-name main -o %t/main
 // RUN: %target-codesign %t/main
 // RUN: %target-run %t/main | %FileCheck %s
 
@@ -160,7 +160,9 @@ func testMixture() async {
   await test { try await testSyncDoesntThrowThenAsyncDoesntThrow() }
 }
 
-runAsyncAndBlock {
-  await testRecursion()
-  await testMixture()
+@main struct Main {
+  static func main() async {
+    await testRecursion()
+    await testMixture()
+  }
 }