turns out that a field or base needs to be laid out in the tail padding of
the base, CGRecordLayoutBuilder::ResizeLastBaseFieldIfNecessary will convert
it to an array of i8.

I've audited the new test results to make sure that they are still valid. I've
also verified that we pass a self-host with this change.

This (finally) fixes PR5589!

llvm-svn: 129673