filter out derive helper attributes during ast lowering but keeping them for rustdoc

[kantnero]

Relands #153102 with an actually_doc guard so that derive helper attributes are preserved in rustdoc JSON output. When running normally, they are dropped into dropped_attributes as before

The problem: @scrabsha 's original implementation dropped derive helper attributes during AST lowering, which is correct for normal compilation. However, rustdoc json relies on these attributes to expose them to tooling (BuFFI). This caused a regression reported in #157107 which was fixed by reverting PR in #157150

The fix: Before dropping an attribute into dropped_attributes, check self.sess.opts.actually_rustdoc. If rustdoc is running, keep the attribute so it appears in the JSON output. Otherwise drop it as before.

r? @JonathanBrouwer
cc @scrabsha

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

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