Revert "UBSan: Teach isDerivedFromAtOffset and findBaseAtOffset about vbases"

This reverts commit r221445. This change leads to false positives
reports from -fsanitize=vptr. See original commit thread for more
details.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224972 91177308-0d34-0410-b5e6-96231b3b80d8