[clang] ms-abi: member pointer inheritance model lock-down fix (#145958)
Lock down the inheritance model for member pointers even when converting
from nullptr.
This fixes a regression introduced in
https://github.com/llvm/llvm-project/pull/131966
There are no release notes, since the regression was never released.
Fixes https://github.com/llvm/llvm-project/issues/144081