[TSan][libdispatch] Guard test execution on old platforms

[yln]

Summary

dispatch_async_and_wait() was introduced in macOS 10.14. Let's
forward declare it to ensure we can compile the test with older SDKs and
guard execution by checking if the symbol is available. (We can't use
__builtin_available(), because that itself requires a higher minimum
deployment target.) We also need to specify the `-undefined
dynamic_lookup` compiler flag.

[kubamracek]

This looks good to me. Note that I was actually suggesting to avoid the dynamic_lookup + forward declare and requiring that the *SDK* is at least 10.14 to run tests (which is separate from your OS version). But this way it's even more compatible, so I think we should go with it.

compiler-rt/test/tsan/libdispatch/async_and_wait.c

-// RUN: %clang_tsan %s -o %t
+// RUN: %clang_tsan %s -o %t -undefined dynamic_lookup
 // RUN: %run %t 2>&1 | FileCheck %s --implicit-check-not='ThreadSanitizer'
 
 #include "dispatch/dispatch.h"
 
 #include <stdio.h>
 
+// Allow compilation with pre-macOS 10.14 (and aligned) SDKs
+API_AVAILABLE(macos(10.14), ios(12.0), tvos(12.0), watchos(5.0))
+DISPATCH_EXPORT DISPATCH_NONNULL_ALL DISPATCH_NOTHROW
+void dispatch_async_and_wait(dispatch_queue_t queue,
+           DISPATCH_NOESCAPE dispatch_block_t block);
+
 long global;
 
 int main() {
+  // Guard execution on pre-macOS 10.14 (and aligned) platforms
+  if (dispatch_async_and_wait == NULL) {
+    fprintf(stderr, "Done.\n");
+    return 0;
+  }
+
   dispatch_queue_t q = dispatch_queue_create("my.queue", DISPATCH_QUEUE_SERIAL);
   dispatch_semaphore_t s = dispatch_semaphore_create(0);
 
   // Force queue to context switch onto separate thread.
   dispatch_async(q, ^{
     dispatch_semaphore_wait(s, DISPATCH_TIME_FOREVER);
   });
   dispatch_semaphore_signal(s);
 
   global++;
   dispatch_async_and_wait(q, ^{
     // The queue continues to execute on separate thread.  This would cause a
     // race if we had used `dispatch_async()` without the `_and_wait` part.
     global++;
   });
   global++;
 
   fprintf(stderr, "Done.\n");
+  return 0;
 }
 
 // CHECK: Done.