[clang][dataflow] Remove private-field filtering from `StorageLocation` creation.

The API for `AggregateStorageLocation` does not allow for missing fields (it asserts). Therefore, it is incorrect to filter out any fields at location-creation time which may be accessed by the code. Currently, we limit filtering to private, base-calss fields on the assumption that those can never be accessed. However, `friend` declarations can invalidate that assumption, thereby breaking our invariants.

This patch removes said field filtering to avoid violating the invariant of "no missing fields" for `AggregateStorageLocation`.

Differential Revision: https://reviews.llvm.org/D126420