[clang][dataflow] Eliminate duplication between `AggregateStorageLocation` and `StructValue`.

After this change, `StructValue` is just a wrapper for an `AggregateStorageLocation`. For the wider context, see https://discourse.llvm.org/t/70086.

## How to review

- Start by looking at the comments added / changed in Value.h, StorageLocation.h,
  and DataflowEnvironment.h. This will give you a good overview of the semantic
  changes.

- Look at the corresponding .cpp files that implement the semantic changes.

- Transfer.cpp, TypeErasedDataflowAnalysis.cpp, and RecordOps.cpp show how the
  core of the framework is affected by the semantic changes.

- UncheckedOptionalAccessModel.cpp shows how this complex model is affected by
  the changes.

- Many of the changes in the rest of the patch are mechanical in nature.

Reviewed By: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D155446