Add a runtime test for ObjC interop.

Author: jckarter

test/Concurrency/Runtime/Inputs/objc_async.h

@@ -0,0 +1,9 @@
+#include <Foundation/Foundation.h>
+
+@interface Butt: NSObject
+
+- (instancetype _Nonnull)init;
+
+- (void)butt:(NSInteger)x completionHandler:(void (^ _Nonnull)(NSInteger))handler;
+
+@end

test/Concurrency/Runtime/Inputs/objc_async.m

@@ -0,0 +1,15 @@
+#include "objc_async.h"
+#include <stdio.h>
+
+@implementation Butt
+
+- (instancetype)init {
+  return [super init];
+}
+
+- (void)butt:(NSInteger)x completionHandler:(void (^)(NSInteger))handler {
+  printf("starting %ld\n", (long)x);
+  handler(679);
+}
+
+@end

test/Concurrency/Runtime/objc_async.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-clang %S/Inputs/objc_async.m -c -o %t/objc_async_objc.o
+// RUN: %target-build-swift -Xfrontend -enable-experimental-concurrency %import-libdispatch -import-objc-header %S/Inputs/objc_async.h %s %t/objc_async_objc.o -o %t/objc_async
+// RUN: %target-run %t/objc_async | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: libdispatch
+// REQUIRES: objc_interop
+
+import _Concurrency
+
+let task = Task.runDetached {
+  let butt = Butt()
+  let result = await butt.butt(1738)
+  print("finishing \(result)")
+  // FIXME: with a proper task runtime, we should exit naturally once all tasks
+  // complete
+  exit(0)
+}
+
+// FIXME: use the real task runtime facilities
+task.run()
+
+dispatchMain()
+
+// CHECK: starting 1738
+// CHECK-NEXT: finishing 679