[Threading][TSan] Move ThreadSanitizer.cpp into the main runtime.

On Darwin, `RTLD_NEXT` doesn't do what we need it to here, with the
result that if `libswiftCore`'s TSan initializer gets found first,
then `libswift_Concurrency` won't have its initializer called at all,
in spite of us using `RTLD_NEXT` to find the next definition.

Fix this by centralising the initializer in `libswiftCore` instead.

rdar://110665213

lib/Threading/ThreadSanitizer.cpp

@@ -1,49 +0,0 @@
-//===--- 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"
-
-namespace swift {
-namespace threading_impl {
-
-bool tsan_enabled = false;
-void (*tsan_acquire)(const void *) = nullptr;
-void (*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() {
-  tsan_enabled = true;
-  tsan_acquire = (void (*)(const void *))dlsym(RTLD_DEFAULT, "__tsan_acquire");
-  tsan_release = (void (*)(const void *))dlsym(RTLD_DEFAULT, "__tsan_release");
-
-  // Always call through to the next image
-  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