Fux a bug where we were trying to add overriders for non-virtual bases of...

Fux a bug where we were trying to add overriders for non-virtual bases of virtual bases more than once.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97173 91177308-0d34-0410-b5e6-96231b3b80d8