Inhibit all-absent-variant optimization for all enum reprs that inhibit layout optimization, not just repr(C).

[zachs18]

Fixes #146984

[rustbot]

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[theemathas]

Does this need a T-lang FCP?

[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.

[SparrowLii]

This looks fine to me. I'd like at least someone from t-lang to have a aware

r? lang