Run async main code on main thread

This patch moves the actual running of the main code onto the main
thread by making it part of the main actor. This should be more
consistent with what folks are expecting.

Author: etcwilde

stdlib/public/Concurrency/Task.swift

@@ -601,6 +601,7 @@ public func runAsyncAndBlock(_ asyncFun: @escaping () async -> ())
 public func _asyncMainDrainQueue() -> Never
 
 public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
+#if os(Windows)
   Task.runDetached {
     do {
       try await asyncFun()
@@ -609,6 +610,21 @@ public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
       _errorInMain(error)
     }
   }
+#else
+  @MainActor @concurrent
+  func _doMain(_ asyncFun: @escaping () async throws -> ()) async {
+    do {
+      try await asyncFun()
+    } catch {
+      _errorInMain(error)
+    }
+  }
+
+  Task.runDetached {
+    await _doMain(asyncFun)
+    exit(0)
+  }
+#endif
   _asyncMainDrainQueue()
 }