[libcxx] [windows] Fix warning about comparing ints of different signs

[mstorsjo]

Summary

This fixes a warning like this:

warning: comparison of integers of different signs:

    'std::__1::__libcpp_tls_key' (aka 'long') and 'DWORD'
    (aka 'unsigned long') [-Wsign-compare]
if (*__key == FLS_OUT_OF_INDEXES)
    ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~

[EricWF]

LGTM.

src/support/win32/thread_win32.cpp

void __libcpp_thread_sleep_for(const chrono::nanoseconds& __ns)
   // FIXME(compnerd) this should be an alertable sleep (WFSO or SleepEx)
   Sleep(__ms.count());
 }
 
 // Thread Local Storage
 int __libcpp_tls_create(__libcpp_tls_key* __key,
                         void(_LIBCPP_TLS_DESTRUCTOR_CC* __at_exit)(void*))
 {
-  *__key = FlsAlloc(__at_exit);
-  if (*__key == FLS_OUT_OF_INDEXES)
+  DWORD index = FlsAlloc(__at_exit);
+  if (index == FLS_OUT_OF_INDEXES)
     return GetLastError();
+  *__key = index;
   return 0;
 }
 
 void *__libcpp_tls_get(__libcpp_tls_key __key)
 {
   return FlsGetValue(__key);
 }
 
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 {
   if (!FlsSetValue(__key, __p))
     return GetLastError();
   return 0;
 }
 
 _LIBCPP_END_NAMESPACE_STD