[tsan] Add a testcase for a race on a Obj-C instance variable

[kubamracek]

Summary

Let's add a testcase for a race on a Obj-C instance variable.

test/tsan/Darwin/objc-race.mm

+// RUN: %clang_tsan %s -o %t -framework Foundation
+// RUN: %deflake %run %t 2>&1
+
+#import <Foundation/Foundation.h>
+
+#import "../test.h"
+
+@interface MyClass : NSObject {
+  long instance_variable;
+}
+- (void)method:(long)value;
+@end
+
+@implementation MyClass
+
+- (void)method:(long)value {
+  self->instance_variable = value;
+}
+
+@end
+
+int main() {
+  NSLog(@"Hello world.");
+  barrier_init(&barrier, 2);
+  
+  MyClass *my_object = [MyClass new];
+  [my_object method:42];
+  
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    [my_object method:43];
+    barrier_wait(&barrier);
+  });
+  
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    barrier_wait(&barrier);
+    [my_object method:44];
+
+    dispatch_sync(dispatch_get_main_queue(), ^{
+      CFRunLoopStop(CFRunLoopGetCurrent());
+    });
+  });
+  
+  CFRunLoopRun();
+  NSLog(@"Done.");
+  return 0;
+}
+
+// CHECK: Hello world.
+// CHECK: WARNING: ThreadSanitizer: data race
+// CHECK:   Write of size 8
+// CHECK:     #0 -[MyClass method:]
+// CHECK:   Write of size 8
+// CHECK:     #0 -[MyClass method:]
+// CHECK:   Location is heap block
+// CHECK: Done.