Auto merge of #33890 - michaelwoerister:collector-driven-trans, r=eddyb

Drive trans from the output of the translation item collector

This PR changes the way how translation works above the item level. Instead of walking the HIR and calling `trans_item()` on everything encountered (while instantiating monomorphizations on-demand), we now just process the list of translation items generated by the `trans::collector`. Using the collector has the benefit of being able to know the exact set of monomorphizations and symbols before actually running translation, something that is crucial for incremental compilation (but also has [other benefits](https://github.com/rust-lang/rust/pull/33602)).

The collector has existed for quite a while now, but so far it's output was only used for running some auto-tests. With this PR it becomes the only source of truth about what gets translated.

One modification we had to make, compared to the initial approach, is that closures are not represented as their own `TransItems`. Doing so, while still supporting non-MIR-based translation, would have been prohibitively complex, and not worth the trouble since legacy-trans will disappear sooner or later. Once there is solely MIR-trans, it would be a good idea to make closures `TransItems` again.

This PR removes the most obvious functions and tables that are not needed anymore, but there's definitely still more cleanup possible later on (e.g. `monomorphize::monomorphic_fn()` does very little at this point). Since there are already more than 10 commits in here, doing this in a separate PR seems to be a better idea.

These changes definitely warrant a crater run.

Thanks @Aatch, for taking on one of the more tedious tasks during the dev-sprint!
Thanks @eddyb, for doing some nice refactorings to symbol name generation and making sure these landed so I could use them!

cc @rust-lang/compiler
cc @rust-lang/tools