[Clang][MS] Remove assertion on BaseOffset can't be smaller than Size.

This assertion triggered when we have two base classes sharing the same offset
and the first base is empty and the second class is non-empty.
Remove it for correctness.

I can't add a test case for this because -foverride-record-layout doesn't read
base class info at all. I can add that support later for testing if needed.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D152472