More tests and cleanup

Adding execution and death test to ensure that we crash appropriately
when the main function throws an uncaught exception, and that the async
main runs correctly.

Also switching to doing the CFRunLoopRun lookup with `RTLD_DEFAULT`
since `RTLD_SELF` isn't available on Linux.

Switching to `try await` since `await try` is no longer the right way to
do that.

Using exit(0) instead of EXIT_SUCCESS since the C++ importer doesn't
mark imported macros with @actorIndependent yet.

Author: etcwilde

stdlib/public/Concurrency/Task.cpp

@@ -535,7 +535,7 @@ void swift::swift_continuation_logFailedCheck(const char *message) {
 
 void swift::swift_task_asyncMainDrainQueue() {
   auto runLoop =
-      reinterpret_cast<void (*)(void)>(dlsym(RTLD_SELF, "CFRunLoopRun"));
+      reinterpret_cast<void (*)(void)>(dlsym(RTLD_DEFAULT, "CFRunLoopRun"));
   if (runLoop)
     runLoop();
   else

stdlib/public/Concurrency/Task.swift

@@ -329,8 +329,8 @@ public func _asyncMainDrainQueue() -> Never
 public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
   let _ = Task.runDetached {
     do {
-      await try asyncFun()
-      exit(EXIT_SUCCESS)
+      try await asyncFun()
+      exit(0)
     } catch {
       _errorInMain(error)
     }

test/Concurrency/async_main.swift

@@ -1,14 +1,21 @@
 // RUN: %target-swift-frontend -dump-ast -enable-experimental-concurrency -parse-as-library %s | %FileCheck %s --check-prefix=CHECK-AST
+// RUN: %target-build-swift -Xfrontend -enable-experimental-concurrency -Xfrontend -parse-as-library %s -o %t_binary
+// RUN: %target-run %t_binary | %FileCheck %s --check-prefix=CHECK-EXEC
+
 // REQUIRES: concurrency
+// REQUIRES: executable_test
 
-func asyncFunc() async { }
+func asyncFunc() async {
+  print("Hello World!")
+}
 
 @main struct MyProgram {
   static func main() async {
     await asyncFunc()
   }
 }
 
+// CHECK-EXEC: Hello World!
 
 // CHECK-AST-LABEL: "main()" interface
 // CHECK-AST:       (await_expr type='()'

test/Concurrency/async_main_throws_prints_error.swift

@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -Xfrontend -enable-experimental-concurrency -Xfrontend -parse-as-library %s -o %t/main
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main  > %t/log 2>&1 || true
+// RUN: %FileCheck %s < %t/log
+
+// REQUIRES: concurrency
+// REQUIRES: executable_test
+
+enum Err : Error { case noGood }
+
+func asyncFunc() async throws {
+  throw Err.noGood
+}
+
+// CHECK: Fatal error: Error raised at top level: main.Err.noGood
+@main struct MyProgram {
+  static func main() async throws {
+    try await asyncFunc()
+  }
+}