[libcxx] [test] Remove MSVC workarounds

[STL_MSFT]

Summary

MSVC has finally fixed the compiler bug that was warning about unreferenced parameters when invoking destructors, so we can remove the void casts that were working around the bug. Yay!

[EricWF]

r283580.

test/support/test_allocator.h

 #if TEST_STD_VER < 11
     void construct(pointer p, const T& val)
         {::new(static_cast<void*>(p)) T(val);}
 #else
     template <class U> void construct(pointer p, U&& val)
         {::new(static_cast<void*>(p)) T(std::forward<U>(val));}
 #endif
     void destroy(pointer p)
-        {
-            p->~T();
-            ((void)p); // Prevent MSVC's spurious unused warning
-        }
+        {p->~T();}
     friend bool operator==(const test_allocator& x, const test_allocator& y)
         {return x.data_ == y.data_;}
     friend bool operator!=(const test_allocator& x, const test_allocator& y)
         {return !(x == y);}
 };
 
 template <class T>
 class non_default_test_allocator
public:
     { ::new((void*)p) Tag_X(Ctor_Tag{}, std::forward<Args>(args)...); }
 
     template<typename U, typename... Args>
     void construct(U* p, Args&&... args)
     { ::new((void*)p) U(std::forward<Args>(args)...); }
 
     template<typename U, typename... Args>
     void destroy(U* p)
-    {
-        p->~U();
-        ((void)p); // Prevent MSVC's spurious unused warning
-    }
+    { p->~U(); }
 };
 
 template<typename T, typename U>
 bool
 operator==(const TaggingAllocator<T>&, const TaggingAllocator<U>&)
 { return true; }
 
 template<typename T, typename U>
 bool
 operator!=(const TaggingAllocator<T>&, const TaggingAllocator<U>&)
 { return false; }
 #endif
 
 
 #endif  // TEST_ALLOCATOR_H