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

This reverts commit r215137.

This doesn't work at all, an offset-offset is probably different than an
offset.  I'm scared that this passed our normal test suite.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215141 91177308-0d34-0410-b5e6-96231b3b80d8