Silence another occurrence of MSVC's suprious unused warning. Patch from STL@microsoft.com

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@277572 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/test_allocator.h

@@ -291,7 +291,10 @@ public:
 
     template<typename U, typename... Args>
     void destroy(U* p)
-    { p->~U(); }
+    {
+        p->~U();
+        ((void)p); // Prevent MSVC's spurious unused warning
+    }
 };
 
 template<typename T, typename U>