[clang][dataflow] Introduce `getFieldValue()` test helpers.

These insulate tests against changes to the `getChild()` functions of `AggregateStorageLocation` and `StructValue` that will happen as part of the migration to strict handling of value categories (see https://discourse.llvm.org/t/70086 for details):

- `AggregateStorageLocation::getChild()` will soon return a `StorageLocation *`
  instead of a `StorageLocation &`. When this happens, `getFieldValue()` will be
  changed to return null if `AggregateStorageLocation::getChild()` returns null;
  test code will not need to change as it should already be checking whether the
  return value of `getFieldValue()` is null.

- `StructValue::getChild()` will soon return a `StorageLocation *` instead of a
  `Value *`. When this happens, `getFieldValue()` will be changed to look up the
  `Value *` in the `Environment`. Again, test code will not need to change.

The test helpers will continue to serve a useful purpose once the API changes are complete, so the intent is to leave them in place.

This patch changes DataflowEnvironmentTest.cpp and RecordOpsTest.cpp to use the test helpers. TransferTest.cpp will be changed in an upcoming patch to help keep patch sizes manageable for review.

Depends On D154934

Reviewed By: ymandel, xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D154935