Rollup merge of #161259 - mejrs:attr_structures, r=JonathanBrouwer

move some attribute related structs out of rustc_attr_ir

This changes it so that

- rustc_session no longer depends on rustc_attr_ir
- rustc_attr_ir no longer depends on rustc_target (which is a fairly sizeable crate)
- rustc_attr_ir and rustc_target can compile in parallel

Previous graph:
<img width="985" height="512" alt="image" src="https://github.com/user-attachments/assets/cbde2414-043e-45f2-88a2-f77a5c9df60c" />

Graph with this PR:
<img width="1248" height="474" alt="image" src="https://github.com/user-attachments/assets/e0e13794-b17b-4337-8768-f0191cb1c0e6" />

r? @JonathanBrouwer