Fix another regression from the "skip vtable pointer initialization"

Fix another regression from the "skip vtable pointer initialization"
optimization. Make sure to require a vtable when trying to get the address
of a VTT, otherwise we would never end up emitting the VTT.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131400 91177308-0d34-0410-b5e6-96231b3b80d8