Build fix for gcc builtin

The __sync_add_and_fetch() builtin parameter is volatile but clang has
'different' type checking and ends up accepting this code.

Undo the C++ cast from r198505 to get libc++/LLVM building with g++ while this
is investigated.

llvm-svn: 199494

libcxx/src/stdexcept.cpp

@@ -79,7 +79,7 @@ __libcpp_nmstr::operator=(const __libcpp_nmstr& s) _NOEXCEPT
     const char* p = str_;
     str_ = s.str_;
     __sync_add_and_fetch(&count(), 1);
-    if (__sync_add_and_fetch(reinterpret_cast<const count_t*>(p-sizeof(count_t)), count_t(-1)) < 0)
+    if (__sync_add_and_fetch((count_t*)(p-sizeof(count_t)), count_t(-1)) < 0)
         delete [] (p-offset);
     return *this;
 }