[PATCH] Fix for libc++ bug #20096 http://llvm.org/bugs/show_bug.cgi?id=20096

[Agustín Bergé]

The existing implementation is the correct one for C++11, where `is_destructible<T>` is defined as:

> For a complete type `T` and given `template <class U> struct test { U u; };`, `test<T>::˜test()` is not deleted.

http://reviews.llvm.org/D4306