[Threading][TSan] Fix builds where TSan isn't supported.

On builds where TSan isn't supported, don't include any code in
ThreadSanitizer.cpp.

rdar://110665213

Author: al45tair

include/swift/Threading/ThreadSanitizer.h

@@ -25,6 +25,9 @@
 namespace swift {
 
 #if defined(_WIN32) || defined(__wasi__) || !__has_include(<dlfcn.h>)
+
+#define SWIFT_THREADING_TSAN_SUPPORT 0
+
 namespace tsan {
 
 inline bool enabled() { return false; }
@@ -34,6 +37,8 @@ template <typename T> T *release(T *ptr) { return ptr; }
 } // namespace tsan
 #else
 
+#define SWIFT_THREADING_TSAN_SUPPORT 1
+
 // If we're static linking to libswiftThreading.a, these symbols can come
 // from there.  If, on the other hand, we're dynamically linked, we want
 // to get them from libswiftCore.dylib instead.

lib/Threading/ThreadSanitizer.cpp

@@ -18,6 +18,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/Threading/ThreadSanitizer.h"
+
+#if SWIFT_THREADING_TSAN_SUPPORT
+
 #include "swift/shims/Visibility.h"
 
 #include <cstdio>
@@ -50,7 +53,9 @@ void __tsan_on_initialize() {
     next_init();
   }
 }
-#endif
+#endif // __has_include(<dlfcn.h>)
 
 } // namespace threading_impl
 } // namespace swift
+
+#endif // SWIFT_THREADING_TSAN_SUPPORT