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

[Eric Fiselier]

Hi Marshall, 

Would you be able to explain why AbstractDestructor should not be destructible?

My reading of the standard seems to suggest that because AbstractDestructor should be destructible because " decltype(_VSTD::declval<_Tp1&>().~_Tp1())" only has to be well formed in an un-evaluated context. It seems to me that invoking the destructor is well formed in this case.

Please let me know if I've made any mistakes.

http://reviews.llvm.org/D4306