[clang][dataflow] Various refactorings in TypeErasedDataflowAnalysisTest.cpp

These simplify the code in their own right, but they are also useful in that they minimize the number of changes that will need to be made when then API of `AggregateStorageLocation` and `StructValue` changes as part of the migration to strict handling of value categories (see https://discourse.llvm.org/t/70086).

Depends On D154949

Reviewed By: xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D154952