Auto merge of #134381 - jdonszelmann:move-attribute-types, r=oli-obk

Split up attribute parsing code and move data types to `rustc_attr_data_structures`

This change renames `rustc_attr` to `rustc_attr_parsing`, and splits up the parsing code. At the same time, all the data types used move to `rustc_attr_data_structures`. This is in preparation of also having a third crate: `rustc_attr_validation`

I initially envisioned this as two separate PRs, but I think doing it in one go reduces the number of ways others would have to rebase their changes on this. However, I can still split them.

r? `@oli-obk` (we already discussed how this is a first step in a larger plan)

For a more detailed plan on how attributes are going to change, see rust-lang/rust#131229

Edit: this looks like a giant PR, but the changes are actually rather trivial. Each commit is reviewable on its own, and mostly moves code around. No new logic is added.