[Debugger Visualizers] Unify *-gnu and *-msvc enum output

[Walnut356]

Followup patch to #145218

Ports the msvc enum synthetic/summary logic to the gnu handlers so they have identical output. Also removes the "look-through-pointers" logic that caused #147450

I know we don't run the test suite in CI anymore, but it's still useful locally so I updated the relevant tests.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.