Turn on the new empty base subobject tracking code. It's a bit faster than the...

Turn on the new empty base subobject tracking code. It's a bit faster than the previous code. However, it still has quadratic performance, something which I intend to fix shortly in a subsequent patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@105161 91177308-0d34-0410-b5e6-96231b3b80d8