[-Wunreachabe-code] Don't warn about unreachable destructors for temporaries.

This can possibly be refined later, but right now the experience
is so incomprehensible for a user to understand what is going on
this isn't a useful warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203336 91177308-0d34-0410-b5e6-96231b3b80d8