[clang][dataflow] Perform deep copies in copy and move operations.

This serves two purposes:

- Because, today, we only copy the `StructValue`, modifying the destination of
  the copy also modifies the source. This is demonstrated by the new checks
  added to `CopyConstructor` and `MoveConstructor`, which fail without the
  deep copy.

- It lays the groundwork for eliminating the redundancy between
  `AggregateStorageLocation` and `StructValue`, which will happen as part of the
  ongoing migration to strict handling of value categories (seeo
  https://discourse.llvm.org/t/70086 for details). This will involve turning
  `StructValue` into essentially just a wrapper for `AggregateStorageLocation`;
  under this scheme, the current "shallow" copy (copying a `StructValue` from
  one `AggregateStorageLocation` to another) will no longer be possible.

Because we now perform deep copies, tests need to perform a deep equality
comparison instead of just comparing for equal identity of the `StructValue`s.
The new function `recordsEqual()` provides such a deep equality comparison.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D153006