[turbopack] Respect VcCellMode in TraitRef::cell (#68473)
*This is a migrated PR. This was in the turbo repository before the
next.js merge.*
**Migrated From:** https://github.com/vercel/turbo/pull/8870
### Description
Building on the `VcCellMode::raw_cell` API from #68467 and the addition
of `ValueType::raw_cell` in #68472, this fixes an edge case where
`TraitRef::cell` didn't respect `VcCellMode` and acted like it was
always using `cell = "new"`.
I noticed this problem while working on local `Vc` resolution. Because
`TraitRef` doesn't know the real concrete type at compile time, this
wasn't possible to fix without these new type-erased `raw_cell` APIs.
The consequences of this bug are mostly just that some data might get
recomputed in certain rare edge cases (most stuff doesn't use
`TraitRef`s heavily).
### Testing Instructions
A regression test is included, which fails without the other changes
included in this PR.
```
cargo nextest r -p turbo-tasks-memory trait_ref
```
