[MS ABI] Lock-in the derived memptr rep. for base-to-derived conversions

We would get this right in the case where an explicit cast was formed
but not when we were performing an implicit conversion.

This fixes PR23828.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239625 91177308-0d34-0410-b5e6-96231b3b80d8