[libcxx] [test] Avoid unary_function.

[STL_MSFT]

Summary

[libcxx] [test] Avoid unary_function.

Replace unary_function inheritance (which was never required,
even in C++98) with argument_type and result_type typedefs.
This increases portability, as unary_function was removed in C++17
and MSVC has implemented that removal.

[EricWF]

I think this would have been a good candidate for post-commit review.

test/support/Counter.h

 };
 
 int Counter_base::gConstructed = 0;
 
 namespace std {
 
 template <class T>
 struct hash<Counter<T> >
-    : public std::unary_function<Counter<T>, std::size_t>
 {
+    typedef Counter<T> argument_type;
+    typedef std::size_t result_type;
+
     std::size_t operator()(const Counter<T>& x) const {return std::hash<T>(x.get());}
 };
 }
 
 #endif