Fix pr17875.

The assert this patch deletes was valid only when aliasing D2 to D1, not when
looking at a base class. Since the assert was in the path where we had already
decided to not produce an alias, just drop it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194411 91177308-0d34-0410-b5e6-96231b3b80d8