Allow dllexport alias to base destructors.

We used to avoid these, but it looks like we did so just because we were
not handling dllexport alias correctly.

Dario Domizioli fixed that, so allow these aliases.

Based on a patch by Dario Domizioli!

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@219206 91177308-0d34-0410-b5e6-96231b3b80d8