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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@221445 91177308-0d34-0410-b5e6-96231b3b80d8