Build on Windows

Windows doesn't have dlsym. Since I don't have a Windows box, I'm just
if-def'ing it out for the time being. Since we don't want Windows going
off and doing dangerous things, I threw in an abort just to be safe.

stdlib/public/Concurrency/Task.cpp

@@ -24,7 +24,9 @@
 
 #include <dispatch/dispatch.h>
 
+#if !defined(_WIN32)
 #include <dlfcn.h>
+#endif
 
 using namespace swift;
 using FutureFragment = AsyncTask::FutureFragment;
@@ -534,10 +536,17 @@ void swift::swift_continuation_logFailedCheck(const char *message) {
 }
 
 void swift::swift_task_asyncMainDrainQueue() {
+#if !defined(_WIN32)
   auto runLoop =
       reinterpret_cast<void (*)(void)>(dlsym(RTLD_DEFAULT, "CFRunLoopRun"));
   if (runLoop)
     runLoop();
   else
     dispatch_main();
+#else
+  // TODO: I don't have a windows box to get this working right now.
+  //       We need to either pull in the CFRunLoop if it's available, or do
+  //       something that will drain the main queue. Exploding for now.
+  abort();
+#endif
 }

test/Concurrency/async_main.swift

@@ -4,6 +4,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: executable_test
+// REQUIRES: OS=macosx || OS=ios
 
 func asyncFunc() async {
   print("Hello World!")

test/Concurrency/async_main_throws_prints_error.swift

@@ -6,6 +6,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: executable_test
+// REQUIRES: OS=macosx || OS=ios
 
 enum Err : Error { case noGood }