Auto merge of #151162 - GuillaumeGomez:cleanup-attr-parsing, r=JonathanBrouwer

Clean up `rustc_attr_parsing`

Follow-up of rust-lang/rust#150934.

It removes the `Option<>` wrapping for `SharedContext::target` field and completely removed the `target_id` field. Considering this type contains a closure and never updates its `target_id` field, there is no need to keep it around, it can be used directly in the lint emitter.

r? @JonathanBrouwer

Author: bors