[libcxx] Disable -Wnon-virtual-dtor warning in <locale>

[EricWF]

Summary

Normally people won't see warnings in libc++ headers, but if they compile with "-Wsystem-headers -Wnon-virtual-dtor" they will likely see issues in <locale>.

In the libc++ implementation time_get' has a private base class, __time_get_c_storage`, with virtual methods but a non-virtual destructor.
time_get itself can safely be used as a polymorphic base class because it inherits a virtual destructor from locale::facet. To placate the compiler we change __time_get_c_storage's destructor from public to protected, ensuring that it will never be deleted polymorphically.

[EricWF]

David Blaikie pointed out that a better way to solve this warning is to make __time_get_c_storage's destructor protected.

include/locale

     virtual const string_type& __r() const;
     virtual const string_type& __x() const;
     virtual const string_type& __X() const;
+
+    _LIBCPP_ALWAYS_INLINE
+    ~__time_get_c_storage() {}
 };
 
 template <class _CharT, class _InputIterator = istreambuf_iterator<_CharT> >