UBSan: Teach isDerivedFromAtOffset and findBaseAtOffset about vbases

When the __virtual_mask is set, __offset_flags >> __offset_shift yields
an offset into the vtable.  Dereferencing this vtable slot gets us the
vbase offset.

Adjust a test case to verify that this, in fact, works.

Differential Revision: http://reviews.llvm.org/D6074