[clang] Bail out when handling union access with virtual inheritance

An assertion issue that arose when handling union member access with
virtual base class has been addressed. As pointed out by @zygoloid,
there is no need for further derived-to-base analysis in this instance,
so we can bail out upon encountering a virtual base class. Minor
refinement on the function name as we might not be handling a union.

Reported-By: ormris

Fixes: https://github.com/llvm/llvm-project/issues/65982
(cherry picked from commit 660876a4)