is_destructible for function types was mistakenly returning true instead of false.  This is true in both C++11 and C++1y, but has been clarified by the post C++11 LWG issue 2049.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189687 91177308-0d34-0410-b5e6-96231b3b80d8