Debug info: Use record layout to find vbase offsets instead of vtables

This fixes an assertion when generating full debug info in the MS ABI
for classes with virtual bases.

Fixes PR20579.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215137 91177308-0d34-0410-b5e6-96231b3b80d8