Split SpannedTypeVisitor into its own crate, rustc_ty_walk

[joshtriplett]

rustc_privacy pulls in all of rustc_ty_utils, but only wants SpannedTypeVisitor and walk_types. Move those to a small rustc_ty_walk crate.

This allows rustc_privacy to start building much sooner.

[rustbot]

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[mejrs]

You need to bless bootstrap, see https://github.com/rust-lang/rust/blob/main/src/bootstrap/README.md#testing

[joshtriplett]

@rustbot ready

[mejrs]

I think we should consider splitting up rustc_ty_utils (perhaps poorly named if it's just providing queries now?) further:

crate A: Queries that need rustc_trait_selection
crate B: Queries that only need rustc_middle (or perhaps rustc_infer - haven't thought about what the best split is here)
crate C: What this pr calls rustc_ty_walk - the actual util stuff shared between A and B and other crates. Maybe there's also stuff currently defined in rustc_middle that could be moved here.

[joshtriplett]

@mejrs That sounds reasonable. But also, I'm a big fan of incremental steps; we can always split/merge crates further after this. :)

[mejrs]

@mejrs That sounds reasonable. But also, I'm a big fan of incremental steps; we can always split/merge crates further after this. :)

Yes! Please keep this PR small, I'm not suggesting it is done right here and now (hence, just "considering" 🙂).

[rust-bors]

☔ The latest upstream changes (presumably #160411) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.