Auto merge of #143460 - JonathanBrouwer:cfg_parser, r=jdonszelmann

Port `#[cfg]` to the new attribute parsing infrastructure

Ports `#[cfg]` to the new attribute parsing infrastructure for https://github.com/rust-lang/rust/issues/131229#issuecomment-2971353197

I've split this PR into commits for reviewability, and left some comments to clarify things