[clang][dataflow] Add a test for a struct that is directly self-referential through a reference.

The ongoing migration to strict handling of value
categories (see https://discourse.llvm.org/t/70086) will change the way we
handle fields of reference type, and I want to put a test in place that makes
sure we continue to handle this special case correctly.

Depends On D154420

Reviewed By: gribozavr2, xazax.hun

Differential Revision: https://reviews.llvm.org/D154421