[Threading][TSan] Rearrange things again.

We need ThreadSanitizer.cpp in libswiftCore for the runtime case, but
we also need it in libswiftThreading for non-runtime cases.

rdar://1106655213

lib/Threading/ThreadSanitizer.cpp

@@ -0,0 +1,59 @@
+//===--- ThreadSanitizer.cpp - Thread Sanitizer support -------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Helper functions for code that needs to integrate with the thread
+// sanitizer.  In particular, TSan can't see inside the runtime libraries,
+// so we occasionally need to give it a hint that we're doing synchronization
+// in order to avoid false positives.
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Threading/ThreadSanitizer.h"
+#include "swift/shims/Visibility.h"
+
+#include <cstdio>
+
+namespace swift {
+namespace threading_impl {
+
+extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
+bool _swift_tsan_enabled = false;
+extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
+void (*_swift_tsan_acquire)(const void *) = nullptr;
+extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
+void (*_swift_tsan_release)(const void *) = nullptr;
+
+#if __has_include(<dlfcn.h>)
+#include <dlfcn.h>
+
+// The TSan library code will call this function when it starts up
+extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
+void __tsan_on_initialize() {
+  _swift_tsan_enabled = true;
+  _swift_tsan_acquire = (void (*)(const void *))dlsym(RTLD_DEFAULT,
+                                                      "__tsan_acquire");
+  _swift_tsan_release = (void (*)(const void *))dlsym(RTLD_DEFAULT,
+                                                      "__tsan_release");
+
+  // Always call through to the next image; this won't work on macOS, but it's
+  // important on Linux to allow others to hook into the thread sanitizer if
+  // they wish.
+  void (*next_init)(void);
+  next_init = (void (*)(void))dlsym(RTLD_NEXT, "__tsan_on_initialize");
+  if (next_init) {
+    next_init();
+  }
+}
+#endif
+
+} // namespace threading_impl
+} // namespace swift