Rollup merge of #140307 - mejrs:condition_parser, r=nnethercote

Refactor rustc_on_unimplemented's filter parser

Followup to https://github.com/rust-lang/rust/pull/139091; I plan on moving most of this code into `rustc_attr_parsing` at some point, but want to land this separately first.

I have taken care to preserve the original behavior as much as I could:
- All but one of the new error variants are replacements for the ones originally emitted by the cfg parsing machinery; so these errors are not "new".
- the `InvalidFlag` variant is new, this PR turns this (from being ignored and silently doing nothing) into an error:
    ```rust
    #[rustc_on_unimplemented(on(something, message = "y"))]
    //~^ ERROR invalid boolean flag
    //~^^ NOTE expected one of `crate_local`, `direct` or `from_desugaring`, not `something`
    trait InvalidFlag {}
    ```
    This does not occur anywhere except in this test. I couldn't find a way that I liked to keep allowing this or to do nothing, erroring was the cleanest solution.
- There are a bunch of FIXME throughout this and the previous PR, I plan on addressing those in follow up prs..

Finally, this gets rid of the "longest" dependency in rustc:
![image](https://github.com/user-attachments/assets/3c3eb3a0-b7b3-40d9-aada-a752e28c8678)

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