[SourceKit] Add missing includes

`ThreadSafeRefCntPtr.h` references `std::enable_if` and `std::move`,
but does not include `<type_traits>` or `<utility>`. This results
errors when compiling on Linux.

Author: modocache

tools/SourceKit/include/SourceKit/Support/ThreadSafeRefCntPtr.h

@@ -16,6 +16,8 @@
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/Support/Mutex.h"
 #include <atomic>
+#include <type_traits>
+#include <utility>
 
 namespace SourceKit {