[tsan] Add a test for "external" API that checks the dup suppression is based on the caller PC

[kubamracek]

Summary

We need to make sure that the "external" API isn't dup'ing all data races into a single one (because the stack might look the same) and suppressing all external races. This works now, so just adding a test for that.

test/tsan/Darwin/external-dups.cc

+// RUN: %clangxx_tsan %s -o %t
+// RUN: %deflake %run %t 2>&1 | FileCheck %s
+
+#include <thread>
+
+#import "../test.h"
+
+extern "C" {
+void *__tsan_external_register_tag(const char *object_type);
+void *__tsan_external_assign_tag(void *addr, void *tag);
+void __tsan_external_read(void *addr, void *caller_pc, void *tag);
+void __tsan_external_write(void *addr, void *caller_pc, void *tag);
+void __tsan_write8(void *addr);
+}
+
+void *tag;
+
+__attribute__((no_sanitize("thread")))
+void ExternalWrite(void *addr) {
+  __tsan_external_write(addr, __builtin_return_address(0), tag);
+}
+
+int main(int argc, char *argv[]) {
+  barrier_init(&barrier, 2);
+  tag = __tsan_external_register_tag("HelloWorld");
+  fprintf(stderr, "Start.\n");
+  // CHECK: Start.
+   
+  for (int i = 0; i < 4; i++) {
+    void *opaque_object = malloc(16);
+    std::thread t1([opaque_object] {
+      ExternalWrite(opaque_object);
+      barrier_wait(&barrier);
+    });
+    std::thread t2([opaque_object] {
+      barrier_wait(&barrier);
+      ExternalWrite(opaque_object);
+    });
+    // CHECK: WARNING: ThreadSanitizer: race on a library object
+    t1.join();
+    t2.join();
+  }
+  
+  fprintf(stderr, "First phase done.\n");
+  // CHECK: First phase done.
+
+  for (int i = 0; i < 4; i++) {
+    void *opaque_object = malloc(16);
+    std::thread t1([opaque_object] {
+      ExternalWrite(opaque_object);
+      barrier_wait(&barrier);
+    });
+    std::thread t2([opaque_object] {
+      barrier_wait(&barrier);
+      ExternalWrite(opaque_object);
+    });
+    // CHECK: WARNING: ThreadSanitizer: race on a library object
+    t1.join();
+    t2.join();
+  }
+
+  fprintf(stderr, "Second phase done.\n");
+  // CHECK: Second phase done.
+}
+
+// CHECK: ThreadSanitizer: reported 2 warnings