[libc++][test] Change IsSmallObject's calculation for std::any's small object buffer

`sizeof(std::any) - sizeof(void*)` is correct for both libc++ and the MSVC standard library.

Differential Revision: https://reviews.llvm.org/D68756

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374407 91177308-0d34-0410-b5e6-96231b3b80d8

Author: CaseyCarter

test/support/any_helpers.h

@@ -26,7 +26,7 @@ namespace std { namespace experimental {} }
 template <class T>
   struct IsSmallObject
     : public std::integral_constant<bool
-        , sizeof(T) <= (sizeof(void*)*3)
+        , sizeof(T) <= sizeof(std::any) - sizeof(void*)
           && std::alignment_of<void*>::value
              % std::alignment_of<T>::value == 0
           && std::is_nothrow_move_constructible<T>::value