[clang][dataflow] Fix initializing a reference field with an `InitListExpr`.

I added a test for this as the ongoing migration to strict handling of value categories (see https://discourse.llvm.org/t/70086) will change the code that handles this case. It turns out we already didn't handle this correctly, so I fixed the existing implementation.

Depends On D154961

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D154965